URSI White Paper on
Solar Power Satellite (SPS) Systems
andReport of the URSI Inter-Commission
Working Group on SPS

June 2007

URSI Inter-commission Working Group on SPS

PREFACE to This BookURSI has established an inter-commission working group (ICWG)
on SPS in 2002. The ICWG worked for the first three years to
prepare a white paper. Only its summary is separated, called White
Paper, and has been thoroughly discussed within the Board since2005. It was approved by the scientific commissions and the national
commissions, and published in Radio Science Bulletin, No. 321, pp.
13-27, June 2007. The rest is compiled as Report of the URSI ICWG
on SPS which is composed of the main text and Appendices which
supply detailed technical and scientific information. This book is acombined version of the URSI White Paper and its supplement. We
hope it will be helpful in facilitating the first step of technical andscientific discussion from both pro and con sides.
Chair of the ICWG on SPS1
Hiroshi Matsumoto
Kyoto University
Secretary to the ICWG on SPS
Kozo Hashimoto
Kyoto University

1

URSI Past President
i

iiPREFACE to URSI White Paper on SPS[From the Editor’s introduction to the issue of the Radio Science
Bulletin in which the White Paper was published (W. R. Stone, Radio
Science Bulletin, No. 321, June 2007, p. 3).]Since the last triennium, the URSI Inter-Commission Working
Group on SPS and the URSI Board have been working on writing
and reviewing an URSI White Paper on Solar Power Satellites. A
solar power satellite (SPS) system uses a satellite to capture powerfrom the sun, generate electricity in orbit, and transmit it to theground using electromagnetic waves. The white paper that appears inthis issue is the result of more than three years of work, including
input and consultation from the URSI Commissions and Member
Committees and the Board. The result is an important document
containing a great deal of information, and makes for very interestingreading. It should also be recognized that there are almost certainly
scientists active in URSI who may disagree with parts of this white
paper. As noted in the White Paper, URSI white papers “aredocuments issued by URSI scientific experts on controversial
subjects involving aspects of radio science. Although under the
responsibility of URSI, they do not necessarily reflect all the viewsof individual URSI Member Committees nor Commissions.” Moreon the nature of URSI white papers and the process associated withgenerating and reviewing them is given in Appendix 1 of the SPS
White Paper. For those interested in more information on solar powersatellite systems, the Report of the URSI Inter-Commission Working
Group on SPS, a document separate from the SPS White Paper,
should be available on the URSI Web site (http://www.ursi.org) by
the time this issue of the Bulletin reaches you.
As noted above, the preparation of this SPS White Paper involved
a very large number of URSI radio scientists. The efforts of themembers of the Inter-Commission Working Group on SPS and the
URSI Board must be particularly acknowledged. Hiroshi Matsumoto
and Kozo Hashimoto played leadership roles within the Working
Group, and Kristian Schlegel provided leadership within the Board inbringing this white paper to completion.

iii

iv

PREFACE to Report of the URSI ICWG on SPS
This Report published by URSI (Union Radio ScièntifiqueInternationale) is to provide a scientific background for discussionsof issues related to Solar Power Satellites. URSI is a non-profit,
non-governmental international union of scientists and engineers
devoted to all aspects of radio science. Since its foundation in 1919,
URSI has been a member of the International Council of Science(ICSU). A vast amount of knowledge and experience in radio sciencehas been accumulated within URSI. Therefore it is appropriate thatURSI be the organization to address all issues related to radio
science.
While the world demand for energy has increased to dramaticlevels, the carbon dioxide emissions from fossil fuels for energy
production turns out a main cause of the global warming. Therefore
it may be a good timing to give this overview of Solar Power
Satellites (SPS) which has been proposed and investigated bothtechnically and theoretically as a potentially clean energy source
though there may be other issues related to radio use. URSI is theappropriate international organization to address all questions andproblems related to radio science aspects of SPS. These problems
and questions are explained and discussed in this document. Aspects
not related to radio science, like launch and transport and other spacetechnologies will only be briefly treated.
It should be stressed that URSI does not unanimously advocate
SPS. Within URSI there are both advocates of SPS voices of concern
and severe reservation. URSI sees its role in providing the necessary
scientific background and a forum for unbiased discussion ofadvantage and disadvantages of SPS. URSI has established aninter-commission working group (ICWG) on SPS in 2002. The
ICWG worked for the first three years to prepare this white paper.
Since 2005, only its summary is separated, called White Paper,
thoroughly discussed within the Board, approved by the scientificcommissions and the national commissions, and published in Radio
Science Bulletin. The rest is this document with Appendices andsupplies detailed technical and scientific information. We hope thewhite paper will be helpful in facilitating the first step of technicaland scientific discussion from both pro and con sides.
The Report is composed of a main text and Appendices. A fullvversion including the White Paper is available in a CD format, while
the first three parts without Appendices are available in a printed
version.
Finally, the editors thank the URSI Board of officers, especially, K.
Schlegel and F. Lefeuvre, W. R. Stone, and P. Lagasse for theirencouragement. They are deeply indebted to N. Shinohara and S.
Kawasaki, J. Mankins, N. Suzuki, and L. Summerer for writing many
parts of chapter and/or appendices. They are also grateful to Y.
Rahmat-Samii, T. Itoh, M. T. Rietveld, M. Davis, J. Lin, Q. Balzano,Y. Omura, and T. Mitani, for writing some sections of this document.
M. Inoue, R. Schillizzi, D. Emerson, M. Ohishi, A. R. Thompson,and W. van Driel have contributed from a view point of radio
astronomy. The editors also thank P. Degauque, F. Lefeuvre, K.
Schlegel, P. Wittke, R. M. Dickinson, T. Takano, M. Taki, D. Preble,
H. Matsuoka, and K. Hughes for their useful comments and C. Kita
for final formatting. Appendix D is a translation of reports inJapanese of JAXA SSPS committee chaired by H. Matsumoto andsome parts are used for the main texts. They thank M. Mori, H.
Nagayama, and Y. Saito for administration, K. Tanaka, M. Oda, S.
Sasaki, M. Utashima, N. Shinohara, K. Hashimoto T. Yoshida, M.
Imaizumi, S. Toyama, H. Kawasaki, T. Yasutake, K. Suzuki T.
Yoshida, M. Imaizumi, S. Toyama, T. Yasutake, K. Suzuki, I.
Matsuoka, and H. Kawasaki for their original writing and checking
the translation, H. Kojima and H. Usui for preparation, and G. Maeda
for English translation. The members of ICWG on SPS are A. C.
Marvin, Y. Rahmat-Samii, T. Ohira, T. Itoh, Z. Kawasaki, S. C.
Reising, M. T. Rietveld, N. Shinohara, D. T. Emerson, W. van Driel,and J. Lin.
URSI Past President
Chair of the ICWG on SPS
Hiroshi Matsumoto
Secretary to the ICWG on SPS
Kozo Hashimotovi

CONTENTS of URSI White Paper on SPS
1. Executive Summary...................................................................... 1
2. Solar Power Satellite Systems...................................................... 3
2.1 The SPS Concept
3
2.2 The Aim and Purpose of this White Paper
5
2.3 The History of SPS Research
7
2.4 A Coherent Set of Numerical Values
10
2.5 Economic Issues
12
2.6 Key SPS Technologies
13
3. SPS Radio Technologies ........................................................... 15
3.1 Microwave Power Transmission
15
3.2 Microwave Power Devices
17
3.3 Rectennas
18
3.4 Control and Calibration
19
4. Radio Science Influences and Effects of SPS ........................... 20
4.1 Interaction with the Ionosphere and Atmosphere20
4.2 Compatibility with Other Radio Services and Applications 2 3
4.3 Microwave Power Transmission Effects on Human Health 26
5. Radio Science Influences for Further Studies ........................ 27
6. Acknowledgements ................................................................... 28
7. References ................................................................................. 29
8. Appendix 1: URSI White Papers ............................................ 34
9. Appendix 2: the ten Scientific Commissions of URSI and theirTerms of Reference ....................................................................... 35vii

CONTENTS of Report of the URSI ICWG on SPS
Chapter 1 Background of SPS Research and Development ....... 39
1.1 Scope of human life in the coming hundred years
39
1.2 Energy demand in the next 50 years
39
1.2.1
Predictions of demand and production of fossil
fuels
39
1.2.2
CO2 emissions from fossil fuel use
43
1.3 Kyoto protocol and global warming47
1.4 Sustainable energy resources48
1.4.1
Terrestrial solar energy
49
1.4.2
Hydropower energy
50
1.4.3
Wind energy
50
1.4.4
Biomass energy
50
1.4.5
Geothermal energy
51
1.4.6
Hydrogen energy
52
1.4.7
Ocean thermal energy
52
1.4.8
Tidal power energy
52
1.4.9
Wave energy
52
1.5 Solar power satellite (SPS) as a sustainable power source
51
1.6 Nuclear energy
52
Chapter 2 Solar Power Satellite .................................................... 55
2.1 SPS features55
2.1.1
Basic concept
55
2.1.2
Clean energy source as a CO2-free energy
57
2.1.3
Comparison with terrestrial photovoltaics
57
2.1.4
Economics
60
2.2 SPS systems
61
2.2.1
Space segment
61
2.2.2
Ground segment62
2.3 SPS key technologies
62
2.3.1
Launch and transportation
62
2.3.2
Solar power generation system
67
2.3.3
Thermal control technology
70
2.3.4
Microwave power transmission on SPS
73
2.3.5
Target detection and beam control
82
2.3.6
Rectennas (rectifying antenna) and groundnetwork
85
viii

2.4

SPS research: State of the art
2.4.1
US research
2.4.2
Japanese research
2.4.3
European research
2.4.4
Worldwide activities

89
90
96
104
110

Chapter 3 SPS Radio Technologies............................................. 119
3.1 Microwave power transmission (MPT) technology
120
3.1.1
Use of EM waves to transmit energy
120
3.1.2
Application of Friis transmission equation
122
3.1.3
Characteristics of microwave power transmission
122
3.2 Microwave devices
123
3.2.1
Microwave semiconductors
123
3.2.2
Microwave vacuum tubes
125
3.2.3
Phase shifters and power dividers
126
3.2.4
Microwave devices, circuits and systems
126
3.3 Beam Control
129
3.3.1
Transmission efficiency
129
3.3.2
Phase synchronization among units
130
3.3.3
Retrodirectivity
130
3.3.4
Software retrodirective system
131
3.3.5
Effects of amplitude and phase errors of antenna
array
132
3.3.6
Current antenna technology and future forecast
132
3.4 Rectennas
133
3.5 Measurement and calibration
134
3.5.1
Huge antenna array measurements on the groundand in space
134
3.5.2
Rectenna measurements
134
3.5.3
Self calibration of antenna gain and phase errors
135
3.5.4
SPS antenna test program
135
3.6 Spin-off technologies
139
Chapter 4 Influence and Effects of SPS...................................... 145
4.1 Interaction with space and the atmosphere
145
4.1.1
Atmospheric effects
145
4.1.2
Space plasma effects in the ionosphere147
ix

4.1.3

Effects of electric propulsion on themagnetosphere150
4.2 Compatibility with other radio services and applications
151
4.2.1
Compatibility with other services such as radio
astronomy
152
4.2.2
Reflection and thermal emission from solar cells
154
4.3 MPT effect on human health and bio-effects
155
4.4 Precautionary principle
158
4.5 Summary of pros and cons of SPS
159
4.5.1
Pros of SPS
159
4.5.2
Cons of SPS
159
4.5.3
Other issues of SPS
160
4.5.4
Pros and cons of SPS in Q&A
161
Chapter 5 URSI and SPS ........................................................... 167
5.1 Technologies
167
5.2 Environments
168
Chapter 6 Further Readings ...................................................... 171

Appendices (Contents in the next page)
Appendix A Microwave Power Transmission Activities in
the WorldAppendix B Various SPS Models
Appendix C US Activities (NASA reports)
Appendix D Japanese Activities (JAXA reports)
Appendix E European Activities (ESA reports)x

CONTENTS OF APPENDICES
Appendix A

Microwave Power Transmission Activities
in the world

A.1 Early history
A.2 US Activities
A.3 Canadian Activities
A.4 Japanese Activities
A.5 European Activities

Appendix B

A-1
A-5
A-5
A-6
A-8

Various SPS Models

B.1 Glaser’s SPS concept
B.2 SPS2000
B.3 Solar Disc
B.4 Abacus Reflector Configurations
B.5 NEDO Model
B.6 JAXA Model
B.7 Roadmaps

Appendix C

B-1
B-2
B-3
B-5
B-6
B-6
B-7

US Activities

NASA SPACE SOLAR POWER ACTIVITIES: 1995-2005
Foreword
C--1
C.1 Overview
C--2
C.2 A Brief History of US SPS and SSP Activities (1960s-1970s)
C--6
C.3 The NASA “Fresh Look Study” (1995-1997)
C-10
C.4 The SSP Concept Definition Study (1998)
C-15
C.5 The SSP Exploratory Research & Technology (SERT)Program (1999-2000)
C-16
C.6 National Research Council (NRC) Review (2000-2001)
C-20
C.7 NASA-NSF_EPRI Research (2001-2003)
C-21
C.8 Recent NASA Research and Development in SSP & Related
Technologies (2004-2005)
C-22
C.9 Summary and Conclusions
C-23
C.10 List of Acronyms and Abbreviations
C-42

xi

Appendix D

Japanese Activities (JAXA reports)

D.1 JAXA Models
D.2 Launch and Transportation
D.3 Solar Power Generation
D.4 Thermal Control Technology
D.5 Microwave Power Transmission on SPS
D.6 Rectenna and Ground Segments
D.7 Economics of SPS
D.8 Environmental and Safety Matters
D.9 Study of Laser-based SPS

Appendix E

D--1
D--6
D-10
D-18
D-25
D-37
D-48
D-54
D-59

European activities (ESA reports)

Solar Power from Space – A Space Contribution to Options for21st Century Sustainable Energy Systems
Abstract
E--1
E.1 Introduction
E--1
E.2 Motivation and Frame
E--2
E.3 Objectives
E--5
E.4 European Approach --- Methodology
E--7
E.5 Reference Systems – Terrestrial
E--9
E.6 Reference Systems – Space
E-12
E.7 Comparison Results
E-12
E.8 Conclusions
E-19

COVER: Solar Power Satellite (Artist’s Concept) ©RISH, Kyoto Universityxii

List of Acronyms and Abbreviations
AC
AIA
AIAA
Astronautics
ASEB
BMDO
C&DH
CDS
CED
CIGS
CNES
COP3
CO2
CTE
CW
DC
DOA
DOD
DOE
EDF
EIA
EMI
EOTV
ESA
ESF
ESH
ETO
FCC
FET
FY
GEO
GN&C
GW
HDTV
HEDS
HEMT
HLLV
HTCI

Alternating current
Active Integrated Antenna
American Institute of

Aeronautics

andAeronautics and Space Engineering Board
Ballistic Missile Defense Organization
Command and data-handling
Concept definition study
Cumulated energy demand
Copper indium gallium di-selenide(French) National Center for the Study of Space
Conference of Parties III
Carbon dioxide
Coefficient of Thermal Expansion
Continuous Wave
Direct current
Direction of arrival
Department of Defense
Department of Energy
Electricite de France
Energy Information Agency
Electromagnetic Interference
solar electric propulsion orbital transfer vehicles
European Space Agency
Environmental and safety factors
Environmental safety and health
Earth-to-orbit
Federal Communications Commission
Field effect transistor
Fiscal year
Geostationary Earth orbit
Guidance, navigation and control
Gigawatt = 1,000 MW
High-definition television
Human Exploration and Development of SpaceHigh electron mobility transistors
Heavy lift launch vehicle
HEDS Technology Commercialization Initiative
xiii

IAA
International Academy of Astronautics
IAF
International Astronautical Federation
IAC
International Astronautical CongressIEEE
Institute of Electrical and Electronics Engineers
IEICE
Institute of Electronics, Information andCommunication Engineers
IIASA
International Institute of Applied Systems
Analysis
IPCC
Intergovernmental Panel on Climate Change
ISAS
Institute of Space and Astronautical Science
ISC
Integrated symmetrical concentrator
ISM
Industry, Science and Medical
ISS
International Space Station
ISU
International Space University
ITAR
International Traffic in Arms Regulations
ITU
International Telecommunication Union
IWG
International Working Group
JAXA
Japan Aerospace Exploration Agencyi
JUSPS
Japan-United States Joint Work Shop on Space
Solar Power Systems
kWkilowattkWekilowatt-electrickWhkilowatt-hourkWmkilowatt-mechanicalii
kWpkilowatt-peakiii
LAN
Local Area Network
LEO
Low-Earth orbit
LNG
Liquefied Natural Gas
LH2
Liquid hydrogen
LSP
Lunar solar power
LOTV
Laser-propulsion OTV
LOXLiquid oxygen
MESFET
Metal-semiconductor FET
METS
Microwave Energy Transmission in Space
METI
Ministry of Economy, Trade and Industry (Japan)
MHD
Magnetohydrohynamics
MINIX
Microwave ionosphere nonlinear interaction
experiment
MMIC
Microwave Monolithic Integrated Circuit
MPD
Magnetoplasmadynamic
xiv

MPM
MPT
MSC
MSFC
MW
NASA
NEDO
NOAA
Administration
NRC
OECD
Development
OMB
OTV
PACM
PAE
PCM
PFD
PLV
PMAD
PV
RAMS
PHEMT
RFID
R&D
RF
RLV
RR
R&T
SAR
SCTM
SEE
SEPS
SERT
SHARP
SLI
SM&C
SOI
SoL
SOTV

Microwave Power Module
Microwave power transmission
Model system concept
Marshall Space Flight Center
Mega-watt = 1,000 kW
National Aeronautics and Space Administration
National Energy Development Office (Japan)
National
Oceanic
andAtmospheric
National Research Council
Organization for Economic Cooperation and
Office of Management and Budget
Orbit Transfer Vehicle
Phase and amplitude controlled magnetron
Power added efficiency
Phase controlled magnetron
Power flux density
Personnel Launch Vehicle
Power management and distribution
Photovoltaic
Robotic assembly and maintenance system
Pseudomorphic high electron mobility transistors
Radio frequency identification
Research and development
Radio frequency
Reusable launch vehicle
Radio Regulations
Research and technology
Specific absorption rate
SSP Concept and Technology Maturation
Societe des Electricien et des Electronicien
Solar electric propulsion system
SSP Exploratory Research and Technology
Stationary High Altitude Relay PlatformSpace launch initiative
Structural materials and controls
Silicon on Insulator
Standard of Living
Solar-thermal propulsion OTV
xv

SPG
SPS
SRES
SSP
SSPA
SSPS
SSPW
TIM
TMM
TWTA
UN
UNFCCC
URSI
USEF
Flyer
USGCRP
VLBI
VSWR
WEC
WPT
YAG

Solar power generation
Solar power satellite
Special Report on Emission Scenarios
Space solar powersolid state power amplifierSpace solar power system
Space Solar Power Workshop
Technical interchange meeting
Thermal materials and management
Traveling Wave Tube Amplifier
United Nations
United Nations Framework Convention on
Climate ChangeInternational Union of Radio Scienceiv
Institute for Unmanned Space Experiment Free
US Global Change Research Program
Very long baseline interferometry
Voltage Standing Wave Ratio
World Energy Council
Wireless power transmission
Yttrium aluminum garneti

formerly NASDA; National Administration of Space Development
Agencyii
mechanical kilowatt output of an engine. 1 horsepower (H.P.) = 0.746
kWmiii
a measure of the peak output of a photovoltaic system
iv
Union Radio-Scientifique Internationale

xvi

URSI White Paper on
Solar Power Satellite (SPS) Systems

1. Executive Summary
As a consequence of an ever-increasing world-wide energy
demand and of a need for a “clean” energy source, the solar powersatellite (SPS) concept has been explored by scientists and engineers
in the United States, Japan, and Europe. An SPS constitutes a methodof generating electricity from solar energy using satellites andtransporting it to the ground via electromagnetic waves. Several
candidate systems have been proposed. However, so far no system
has been either constructed or tested in space, and it is currently
unknown when one might be.
The purpose of this URSI white paper is to provide knowledge
about the SPS concept based on evidence, and an open forum for
debate on the scientific, technical, and environmental aspects of the
SPS concept*.
In a typical SPS system, solar energy is collected in space by asatellite in a geostationary orbit. The solar energy is converted todirect current by solar cells, and the direct current is in turn used to
power microwave generators in the gigahertz frequency (microwave)
range. The generators feed a highly directive satellite-borne antenna,
which beams the energy to the Earth. On the ground, a rectifying
antenna (rectenna) converts the microwave energy from the satellite
into direct current, which, after suitable processing, is fed to theterrestrial power grid. A typical SPS unit – with a solar panel area of
about 10 km2, a transmitting antenna of about 2 km in diameter, anda rectenna about 4 km in diameter – may yield an electric-power
output of about 1 GW. Two critical aspects that have motivatedresearch into SPS systems are the lack of attenuation of the solar flux
by the Earth’s atmosphere, and the twenty-four-hour availability ofthe energy, except around midnight during the equinox periods.
Among the key technologies involved in SPS systems aremicrowave generation and transmission techniques, wave*

URSI (International Union of Radio Science) white papers are documents
issued by URSI scientific experts on controversial subjects involving
aspects of radio science. Although under the responsibility of URSI, they
do not necessarily reflect all the views of individual URSI Member
Committees nor Commissions (see Appendix 1).

1

propagation, antennas, and measurement and calibration techniques.
These radio-science issues fall within the scientific domain of theInternational Union of Radio Science (URSI). URSI’s ten ScientificCommissions (Appendix 2) cover a broad range of aspects involvedin an SPS system, ranging from the technical aspects of microwave
power generation and transmission to the effects on humans andpotential interference with communications, remote-sensing, andradio-astronomy observations.
This has led URSI to organise an open forum for the debate of theradio-science aspects of SPS systems and related technical andenvironmental issues. The present white paper is intended to draw
attention to these aspects of SPS systems. It is not URSI’s intention
to advocate solar power satellites as a solution to the world’s
increasing energy demands, or to dwell on areas outside of URSI’s
scientific domain, such as the whole issue of the space engineering tolaunch, assemble, and maintain an SPS system in space, theeconomic justification, and public acceptance. URSI is well awarethat if a practical SPS system is feasible, the realisation of such asystem is far in the future. Many of the required technologies
currently exist, but some of these must be substantially advanced,
and others must be created.
Microwave power transmission is an important technology for
SPS systems, since its overall efficiency is one of the critical factorsthat determines the interest in such systems from an economic
standpoint. Ideally, almost all energy transmitted from thegeostationary orbit should be collected by the rectifying antennas on
the ground. In that respect, an overall dc-to-microwave-to-dc power
efficiency in excess of 50% is needed (see Section 2.4), which
requires the development of suitable microwave power devices.
Accurate control of the antenna beam is essential, and measurement
and calibration are important issues. Even if these technologies canbe successfully developed, there remains the challenging task of
combining the outputs of thousands or even millions of elements toform a focused beam. Proper safety measures have to be developed
to be certain that the transmitted microwave beam remains within therectenna’s area. Maintenance of the space systems may be very
difficult and expensive in the harsh environment of a geostationary
orbit. Ensuring the long-term stability of huge structures in space in2the presence of solar radiation pressure and tidal forces is anunsolved problem.
The influence and effects of electromagnetic emissions from an
SPS, and, in particular, the microwave power transmission, are radioscience issues that concern URSI. Atmospheric effects on the
microwave beam, and linear and non-linear interactions of the
microwave beam with the atmosphere, ionosphere, and spaceplasmas, are among the numerous issues that must be investigatedand evaluated. Undesired emissions – such as harmonics, grating
lobes, and sidelobes from transmitting antennas and rectennas – mustbe sufficiently suppressed. This is true not only to avoid wasting
power, but also to avoid interference with other radio services andapplications and with remote sensing and radio astronomy, in
accordance with the provisions of the Radio Regulations of theInternational Telecommunication Union (ITU). The evaluation of
possible effects on human health and the incorporation of appropriate
safety measures are essential for legal operation and public
acceptance of this power-generation technique.
Finally, this paper identifies specific radio-science issues requiring
further studies. It is stressed that only some of these questions can besolved by laboratory work, simulations, and system analysis. Testingof elements of such large systems in space is mandatory before apossible demonstration SPS unit can be considered, and broadinternational consensus is likely to be required before an SPS
demonstration system can be launched.

2. Solar Power Satellite Systems
2.1 The SPS Concept
A solar power satellite is a very large-area satellite in an
appropriate orbit (see Section 2.6), which would function as anelectric power plant in space. The satellite would consist of threemain parts: a solar-energy collector, to convert solar energy into dc
electric power; a dc-to-microwave converter; and a large antenna
array, to beam the microwave power to the ground. For the
production of 1 GW of dc power, the solar collector would need to3have an area of 10 km2, and would consist of either photovoltaic
cells or solar thermal turbines. The dc-to-microwave converter could
be realised using either a microwave-tube system or a semiconductor
system, or a combination of both. For transmitting the power to theground, frequency bands around 5.8 GHz or 2.45 GHz have been
proposed, which are within the microwave radio windows of theatmosphere. The antenna array to transmit the energy to the groundwould require a diameter of about 2 km at 2.45 GHz, and its beam
direction would have to be controlled to an accuracy of significantly
better than 300 m on the Earth, corresponding to 0.0005°, or lessthan 2 arc seconds (for a geostationary orbit of the satellite).
In addition to the SPS orbiter, a ground-based power-receiving site
has to be constructed, consisting of a device to receive and rectify the
microwave power beam, i.e. to convert it back to dc electric power.
This device is called a rectenna (rectifying antenna). The dimensionsof the rectenna site on the ground depend on the microwave
frequency and the size of the transmitting antenna. A model system,
operating at 2.45 GHz, would use a rectenna site with a diameter of
4 km and a satellite-based transmitting antenna with a diameter of
2 km (see Section 2.4). The peak microwave power-flux density at
the rectenna site would then be 300 W/m2, if a Gaussian power
profile of the transmitted beam is assumed. The beam-intensity
pattern would be nonuniform, with a higher intensity in the centre of
the rectenna and a lower intensity at its periphery. For human safety
requirements, the maximum-allowable microwave power level has
been set to 10 W/m2 in most countries, and the SPS power-flux
density would be constructed to satisfy this requirement at the
periphery of the rectenna. After suitable power conditioning, theelectric output of the rectenna would be delivered to the power
network.
The combination of an SPS in orbit and the ground-based rectenna
will be called an SPS “unit” in the following. On a global scale, avery large number of 1 GW units may be necessary for a practical
SPS system. More details about the SPS concept can be found in [1].
For the sake of completeness, it should be mentioned that besides
microwave power transmission, laser power transmission has also
very recently been suggested [1, Appendix D.9]. In such a scenario,
highly concentrated solar radiation would be injected into the laser
medium (direct solar pumping) and transmitted to Earth. On the4ground, the laser light would be converted to electricity byphotovoltaic cells. It is obvious that such a system would be
fundamentally different from a “classical” SPS using microwave
power transmission: In space, there would be the light-concentration
system and the lasers instead of a photovoltaic-cell array and thetransmitting antenna; on the ground, there would be a photovoltaiccell array instead of the rectenna. Since the technological challenges
and problems for such laser-based systems have not yet been
sufficiently explored, and since many subcomponents are at a low
technology-readiness level, this approach will not be treated in thiswhite paper.

2.2 The Aim and Purpose of this White Paper
There are SPS-related issues that are highly controversial.
Although several space agencies have pursued SPS studies andresearch (see the next section), very critical papers have been
published that concluded that an SPS is impractical and will never go
into operation (e.g., [2]). A more pro-SPS reply to this criticism [3]was based on the economic issues raised in [2]. Among thecontroversial issues is the question of the space engineering andtechnology that are necessary for the launch, and the assembly andthe maintenance of an SPS system, all of which to a great extent are
not yet possible. Other heavily debated issues are related toeconomic justifications (in comparison with other power sources),are related to the question of whether an SPS can provide a base-load“clean” power system on a global scale, are related to military
applications, and are related to public acceptance. All of these issues
are beyond URSI’s scientific domain and will therefore not be
discussed in this white paper. Social issues of an SPS may perhaps
be addressed by the International Council for Science (ICSU).
Instead, this white paper will focus on the radio-science aspects ofan SPS. Among the key radio-science technologies involved in the
SPS concept are microwave generation and transmission techniques,antennas and beam control, and the very challenging task ofprotecting other services to the levels required by the International
Telecommunication Union (ITU). Of the various scientific5organisations or unions concerned with international development
and applications of these technology areas, URSI has an importantrole to play, because it covers most aspects of the above-mentioned
radio techniques. The scientific competence of URSI’s ten
Commissions (see Appendix 2) encompasses aspects of microwave
power generation (Commissions B, C, and D), antennas(Commission B), calibration (Commission A) and transmission(Commissions G and H), the effects of electromagnetic emissions onhumans (Commission K), the potential interference with
communications (Commission C and E), remote-sensing(Commissions E and F) and radio-astronomy (Commission J)
observations, and, to some extent, solar-cell technology(Commission D). Thus, URSI can provide a continuing forum fordevelopment, discussion, and debate on technical issues related to
SPS systems.
In keeping with what has been said above, it is not the intention ofthis document to advocate an SPS as a “clean” solution to theworld’s increasing power demand (as is argued, for instance, in [1]).
However, it is conceivable that an SPS, and, more generally,
microwave power transmission, may be used in the future for special
purposes. Among such possible scenarios are bringing energy toremote areas on the globe that are difficult to otherwise access,sending energy from spacecraft to spacecraft, or providing energy to
the dark side of the moon (in compliance with Recommendation
ITU-R RA.479, recognising a shielded zone on the moon). Possible
spin-offs from SPS-related research have been considered elsewhere[1, Section 3.6].
A number of the issues related to radio science that are addressed
here are also of relevance to the process that the International
Telecommunication Union (ITU) has initiated towards an ITU-R
Recommendation and/or Report on wireless power transmissions, tobe completed by 2010 at the latest [4].
It should be stressed that an SPS is not imminent. Many changes
in technology can be expected before an SPS is launched. Major
technological problems still have to be solved, even before ademonstration project could be realised. On the other hand, the radioscience aspects of an SPS encompass many interesting scientific,
engineering, and technological challenges. To identify, to describe,and to discuss these items is the main aim of this white paper.

62.3 The History of SPS Research
The first concept of an SPS system was proposed by P. Glaser in1968 [5], after a series of experiments on microwave powertransmission [6a, 6b]. Following this article, the United States
conducted an extensive feasibility study in 1978-1980. The
feasibility study was a joint effort of NASA (National Aeronautics
and Space Administration) and the Department of Energy. A
reference model was proposed in 1979, known as the NASA/DoE
reference model (Figure 1, [7]). Research on an SPS was suspended
in the US in 1980, due to high estimated costs. Given a pre-set policy
to re-evaluate the SPS concept after an appropriate time interval, theFresh-Look-SPS concepts were published in 1977 as an improved
SPS reference system. This included the “Sun Tower” SPS concept(Figure 1, [8]). This is a constellation of medium-scale, gravitygradient-stabilised, microwave-transmitting space solar powersystems. Each satellite resembles a large Earth-pointing sunflower, inwhich the face of the flower is the transmitting array, and the“leaves” on the stalk are solar collectors. The Sun Tower is assumed
to transmit at 5.8 GHz from either a low Earth orbit or ageostationary orbit, and to operate sun-synchronously at atransmitted microwave power level of about 200 MW. NASA statedthat due to its extensive modularity, the low-Earth-orbit concept
entails the use of relatively small individual system components,
which could be developed at a moderate price, ground-tested inexisting facilities, and could be demonstrated in a flight environment
during a sub-scale test.
An SPS system using mirrors for sunlight concentration on thesolar cells, the Integrated Symmetrical Concentrator, was also
proposed. It uses 24 or 36 plane mirrors of 500 m diameter for aconcentration factor of two or four (Figure 1, [9]).

7Figure 1. An artist’s impressions of various current SPS models:NASA/DoE SPS Reference Model (top left), Sun Tower (NASA, top
centre) [8], Integrated Symmetrical Concentrator (top right) [9],JAXA 2003 Free Flyer Model (middle left) [18], Tethered-SPS(USEF, middle right) [19], and Sail Tower (ESA, bottom) [10].
The European Space Agency (ESA) proposed a Sail Tower SPS(Figure 1, [10]), the design of which is similar to that of NASA’s
Sun Tower SPS. However, the Sail Tower SPS uses thin-film
technology, and an innovative deployment mechanism developed for
150 m × 150 m solar sails. The power generated in the sail modules
is transmitted through the central tether to the antenna, where
microwaves at 2.45 GHz are generated in mass-produced
inexpensive magnetrons. The energy emitted would be 400 MW. In
2003, the Advanced Concepts Team (ACT) of ESA initiated a threephased, multiyear program related to solar power from space(including laser power-transmission concepts) [11]. In addition, aEuropean Network on Solar Power from Space was established. It

8provides a forum for all relevant and interested European players in
the field of SPS, including industry, academia, and institutions.
Japanese scientists and engineers started their SPS research in theearly 1980s. They conducted a series of microwave powertransmission experiments, such as the world’s first rocket experimentwith powerful microwave transmission in the ionosphere [12, 13],experiments on the ground [14], computer simulations [15],theoretical investigations [16], and system studies for ademonstration experiment [17]. After a conceptual study phase, twoJapanese organisations have recently proposed their own models.
JAXA (Japan Aerospace Exploration Agency) proposed an SPS
5.8 GHz/1 GW model (Figure 1, [18]), which is different from theNASA/DoE model. It is based on a formation flight of a rotating
mirror system and an integrated panel, composed of a photovoltaiccell surface on one side and a phased microwave-array antenna on
the other side. Formation-flying mirrors are used to eliminate theneed for rotary joints. The Institute for Unmanned Space ExperimentFree Flyer (USEF) proposed a simpler model (Figure 1, [19]). The
USEF model is a tethered SPS, which is composed of an integrated
panel similar to JAXA’s, but suspended by multi-tether wires
emanating from a bus system above the panel.
The leading group in Japan in basic SPS-related research is based
at Kyoto University. Many projects on microwave powertransmissions have been conducted, and several important papers
have been published (e.g., [12-16]). To a large extent, this white
paper is based on an extensive review of SPS issues prepared by an
URSI Inter-Commission Working Group [1] under the leadership ofthe Kyoto group.
International collaboration was established at a Japan-US SPS
workshop [20], an International Conference on SPS and Microwave
Power Transmission [21], by the International Astronautical
Congress (IAC) Space Power Committee, and by an URSI InterCommission Working Group.
More details about the different proposed models are available in[1].

9

2.4 A Coherent Set of Numerical Values
A set of typical numerical values was extracted from the various
concepts of SPS mentioned in the previous section. This set forms
the basis of the discussion in this white paper.
Assuming that an SPS unit will generate 1 GW effective power on
the ground, the characteristic efficiencies are summarised in Table 1.
The figures are given for a 2.45-GHz unit; corresponding values fora 5.8-GHz unit are not fundamentally different. Therefore, in order togenerate 1 GW at the ground, one needs to collect about 14 GW in
space. Since the solar radiation power flux is equal to 1.37 kW/m2,
one needs a solar-panel area of approximately 10 km2. The
transmitted RF power is 14 × 0.13 × 0.78 ≈ 1.44 GW. Taking into
account the RF collection efficiency of 87%, the RF power received
at the ground level is P = 1.25 GW. The efficiency of the microwave
power transmission (dc-microwave-dc) is the product of theefficiencies given in lines 2-4 of Table 1, i.e. 54%. (Actually,
54.18% was demonstrated and certified in a NASA laboratory test[22]).
Table 1. The efficiencies for SPS processes (for 2.45 GHz).
Quantity
Solar-power-to-dcpower efficiency
dc-power-to-RF-power
efficiency
RFcollection
efficiency
RF-power-to-dc-power(rectenna)

Efficiency
13%

Reference[1, Section 2.4.1.2]

78%[1, Section 2.4.1.2]

87%[1, Section 2.4.1.2]

80%[1, Section 2.3.6,1](average of 70% and90%).

Total efficiency

7%

In order to define the rectenna characteristics, a reasonable value
for the power flux at the centre of the rectenna system has to be
assumed. Different values have been proposed, between 230 W/m2

10

and 1000 W/m2 [1, Table 2.3.2]. Here, a conservative value of 300
W/m2 (which is less dangerous from a biological point of view: see[1, Section 4.3]) is adopted for the central power flux. Assuming a
Gaussian distribution of the power at the ground, and assuming
further that the power flux at the edge of the rectenna is equal to
10 W/m2 (for safety reasons, which are discussed in [1, Section 4.3]),
after a simple calculation one arrives at a radius of the rectenna ofrr ≈ 2 km. If L is the altitude of the geostationary orbit( L = 3 6 0 0 0 km) and if r t is the radius of the transmitting antenna,
one has that approximately rr L = λ ( 2 rt ) , where λ is the RF
wavelength ( λ = 0.12 m at 2.45 GHz). Therefore, rt = 1080 m (a moreaccurate estimate would arrive at 1200 m: see [1, Section 3.1.1]). The
assumed sizes are summarised in Table 2.
Table 2. The size of the SPS components being considered.
Quantity
Solar-cell array
Transmitting antenna
satellite
RectennaSize
10 km2 area
on 2.4 km diameter (for 2.45 GHz)
4 km diameter (independent of
frequency in the above estimate)

The last number to be introduced is the desired pointing accuracyof the transmitting antenna. In most projects, one assumes that theallowed displacement of the centre of the beam is a small fraction of
the diameter of the rectenna system. In [1, Section 2.1.1], the adopted
value of this displacement was 300 m, so that the required pointing
accuracy for a geostationary power station is 0.0005°. It should be
noted that the above estimate for the rectenna size does not take into
account any safety margin due to the pointing accuracy of 300 m.

11

2.5 Economic Issues
As already stated in Section 2.2, economic-related issues areoutside of URSI’s scientific domain. Some important aspects are
therefore touched on only briefly in this section, with some figures
quoted from the available literature.
There are four main factors that determine the power-production
costs of an SPS system: photovoltaic module efficiency and costs,
mass-specific power production (W/kg) of the solar modules and thetransmission system, microwave power-transmission efficiency, andlaunch costs. The target is an efficiency of about 50% for the total
microwave power transmission dc-microwave-dc conversion (seeSection 3.1), and a specific power output of 1 kW/kg for the whole
microwave power-transmission system. The published SPS costestimates are based on a launch cost of USD150/kg [1,Section 2.1.4]. All these assumptions lead to an estimated energygeneration cost of approximately USD0.1-0.2 per kWh for an SPS
system[23]. These estimates remain controversial. For example,
present-day launch and space-assembly costs are greater than twoorders of magnitude higher than the desired USD150/kg (present-day
launch costs are USD10,000/kg [24]). While NASA expects thelaunch costs to decrease by a factor of 100 by 2025 and by a factor of1000 by 2040 [25], ESA is less optimistic. In a corresponding ESA
report, the energy-generation costs for a 500 GW SPS system were
estimated to be USD0.40/kWh, assuming transportation costs of
USD1,500/kg, and a mass-specific power production of 0.2 kW/kg[26]. In the same report, it was stated that transportation costs may be
reduced to USD200/kg in the future.
A direct comparison of the output power from a space-based solar
power unit with that from a terrestrial photovoltaic array with equalarea is not straightforward. On one hand, a simple estimate of theenergy output yields an advantage of about a factor 2.5 for the SPS.
For the SPS system,
1.37 kW/m2 solar power flux in space × 0.07 overall SPS efficiency(Table 1) × 24 h = 2.3 kWh/m2/day.
For a terrestrial solar-cell array,

12

5 kWh/m2/day average solar power flux at a sunny place (Arizona[27]) × 0.17 solar cell efficiency = 0.85 kWh/m2/day.
On the other hand, a detailed economic comparison of the costs
turns out to be very complicated and dependent on many factors,
such as launch costs (see above), SPS concept, power-consumption
profile (base-load versus non-base-load power-supply systems),
storage technology (for base-load power supply), terrestrial powertransmission system (depending on the location of the terrestrial
power plant), energy payback times, and others. ESA conducted
several corresponding studies (including also terrestrial solar thermal
plants) (e.g., [28, 29]. One of these came to the conclusions that (i)for a base-load power supply, SPS systems above 5 GW and launchcosts between USD824 and USD1023/kg would be required for an
SPS to be competitive with terrestrial plants; (ii) for non-base-load
power supplies, SPS systems above 50 GW and launch costs
between USD206 and USD2146/kg would be required for an SPS toreach a competitive level with terrestrial plants [28]. More-detailed
results of these comparisons are presented and discussed in [1,Section 2.4.3 and Appendices E.5-E.7].

2.6 Key SPS Technologies
The most important key technology concerns the infrastructure tolaunch, assemble, transport, and maintain the SPS system. Since this
topic is beyond URSI’s scientific domain, it will not be dealt withhere.
The key elements in the dc power generation for the SPS system
are the solar cells. Thin-membrane (amorphous) silicon solar cells
are expected to be the most suitable type for the SPS system becauseof their good performance for a given weight, and because ofconservation of natural resources, although their conversion
efficiency is lower than the figures for Si cells (17.3% [7]) and GaAs
cells (20% [7]). Mass-production feasibility is also an important
aspect in choosing the most suitable solar-cell type. A sunlight
concentrator would enhance the power output. Therefore, two typesof power-generation systems have been studied: (a) a massive light-

13

concentration type [9], and (b) a super-light-weight thin-membrane
type [30]. An increase of the total power-conversion efficiency is tobe greatly desired. However, it should be noted that solar cells inspace deteriorate, due to accelerated solar-wind particles and solar
radiation. Radiation-hardened cells are already available for longterm space missions, but at considerably higher costs than cells forterrestrial use.
The thermal design and control of the SPS system will also be ofimportance, particularly if sunlight concentration is applied. One
method for thermal control of the generator is blockage of theinfrared radiation from the sun, either by effective reflection or byband-elimination filters for infrared radiation.
The radio science and technology of an SPS system, such as the
microwave power transmission, microwave power devices,
rectennas, and beam control, will be discussed in detail in Section 3.
A very important detail of an SPS is the proper orbit in space. A
geostationary orbit has been proposed for most of the systems
envisioned so far. However, a more-remote orbit, an L2-halo orbit[31], was also considered. It is generally assumed that the SPS is
assembled at a low Earth orbit, with subsequent transportation to ageostationary orbit. Modern SPS concepts rely on robotic assemblyand maintenance systems, rather than human astronauts for theassembly task. For transportation, suitable orbit-transfer vehicles
have to be developed to transport a very large structure from a lower
to a higher orbit. Solar electric-propulsion orbital-transfer vehicles
have been suggested for this purpose. Some corresponding prototype
propulsion systems, such as a magneto-plasmadynamic thruster, aHall thruster, and a microwave-discharge ion engine, have beentested ([1, Section 2.3.1.2).
It should also be noted that the selection of the final working orbit
of an SPS may have important implications for the antenna design
and its characteristics (far-field or Fresnel region).
Other key issues of SPS technology are lifetime and maintenance.
The limited lifetime of solar cells has already been mentioned, but along-term radiation hazard also exists for any solid-state device on
the SPS, such as dc-to microwave converters, for instance. In
addition, there is the problem of the long-term mechanical stabilityof the very large structures of the solar panels and the microwave
transmitting antenna. The long-term influence of tidal effects and14

radiation pressure have to be examined. In principle, both effects candeform the structure as well as change its orientation. In particular,
the radiation pressure exerts a force that changes continuously indirection with respect to the line joining the satellite and therectenna. This may pose serious problems concerning the control of
the orbit and the orientation of the RF beam. The amplitude of thisforce is of the order of 100 N for a solar-cell area of 10 km2 (2 ×solar radiation power flux × 10 km2/velocity of light). Regarding
maintenance, the present-day experiences for low Earth orbits with
the Hubble space telescope and the International Space Station
indicate that maintaining and servicing a much larger system in amuch higher orbit may be very difficult and much more expensive
than for low Earth orbits. A completely new approach to space
maintenance may be required to maintain assets at geostationary
orbit. Currently, progressive replacement is the only viable option.

3. SPS Radio Technologies
3.1 Microwave Power Transmission
Wireless communication uses radio waves as carriers of
information. However, in the microwave power-transmission system,
radio waves would be used as carriers of energy. In principle, theenergy-carrying microwaves would be monochromatic waves,
without any modulation. The microwave power transmission would
use power densities at the surface of the transmitting antenna that are
three or four orders of magnitude higher than the corresponding
levels in wireless-communication systems, and up to 25 orders ofmagnitude higher than power densities received by the radioastronomy and remote-sensing services.
The main parameters of the microwave power-transmission system
for the SPS system are the frequency, the diameter of thetransmitting antenna, the output power (beamed to the Earth), and the
maximum power-flux density. In addition to the system parametersdescribed above, the weight per unit power of the microwave devices
is also of importance [1, Section 3.2].

15

Efficiency is very important for the microwave powertransmission system. Assuming the SPS transmitting-antenna-torectenna propagation path is optimum, the following efficiencies will
be important: dc-to-radio-frequency (RF) conversion, RF-to-dc
conversion, and beam-collecting efficiencies. Conversion efficiencies
higher than 80% for both RF-dc and dc-RF conversions arenecessary to make the cost of the SPS system reasonable (seeSection 2.4).
Various types of transmitting antennas have been considered, suchas slotted-waveguide antennas, dipole antennas with reflectors, andmicrostrip antennas. The most suitable antenna type depends on thechosen microwave generator and amplifier, but also on weight. A
possible concept seems to be the active integrated antenna technique,combing the dc power generation, microwave conversion, andradiation and control in one multi-layered plate [32].
As mentioned in Section 2.4, the diameter of a transmitting
antenna array of a 1 GW SPS system would be about 2 km. The
average microwave power-flux density at the array of the SPS would
then be about 300 W/m2 on the surface of the transmitting antenna. A
phased antenna array is planned for the SPS system, in order toobtain high-efficiency beam collection under the condition offluctuating SPS attitudes. Depending on the frequency of the
microwave power transmission, e.g. 2.45 GHz or 5.8 GHz, thenumber of antenna elements per square meter would need to be of
the order of 100 or 400, where the power delivered by a singe
element would be 10 W or 2.5 W, respectively [1, Section 3] . Thus,
the total number of elements could be of the order of several
hundreds of millions (this number could be substantially reduced if
single klystrons of more than 1 kW output power were used to feed
one antenna element). Such a large phased array has neither beendeveloped nor constructed up until now, even on Earth. It is
uncertain if simple scaling of already realised arrays is possible, orwhether it may lead to unexpected problems.
Hence, realising the SPS system will require overcoming manyengineering challenges, such as arrays with a dc-RF conversion
efficiency higher than 80%, a phase-shifting system with very low
root-mean-square errors for accurate beam control, phasesynchronisation over millions of elements, and very-low-cost mass
production of these elements.

163.2 Microwave Power Devices
Many possibilities have been proposed for the microwave
generators, such as microwave vacuum tubes (klystrons, magnetrons,
travelling-wave tube amplifiers), semiconductor transmitters, andcombinations of both technologies. These types of generators havebeen compared with respect to their efficiency, output power, weight,
and emitted harmonics [1, Section 2.3.4.2]. The dc-to-RF conversion
efficiency for microwave vacuum tubes can be as high as 65% to75%; the power of a single tube can be more than 100 kW. For
semiconductor transmitters, the best achievable efficiency is 40%,
the power from a single transmitter being below 100 W. Better
efficiencies may be possible with new devices, such as wide-bandgap
devices using GaN, which have significant power output, inparticular at microwave frequencies of 2.45 GHz and 5.8 GHz [1,Section 2.3.4.2 (4)].
Compared to semiconductor technologies, a microwave tube hashigher efficiency, lower cost, and a smaller power-to-weight ratio(kW/kg), even if one includes the power source, the dc-dc converter,
the cooling system, and all the other elements needed to drive thesystem. Some of the SPS concepts are based on a microwave powertransmitter with microwave tubes, such as klystrons and magnetrons.
For example, a new concept for a microwave transmitter has beendeveloped. It is called a phase-controlled magnetron, and it satisfies
both the requirements of high efficiency and beam controllability[33]. A hybrid tube-semiconductor system is also a possible solution
currently under investigation [34].
For the high-efficiency power transmitters, a design that generates
a low amount of harmonics, and low-loss phase shifters, areparticularly important and would need to be developed.
Manufacturability would be one of the important considerations in
the implementation of particular technologies for the microwave
power-transmission system. Since the SPS requires hugeinvestments, even in electronic parts, the availability of particularmaterials and the manufacturability need to be examined. From amanufacturing point of view, recent semiconductor technologies
could be useful for SPS systems. However, their reliability in space17

would need to be investigated. For the microwave powertransmission technology, the reduction of the weight per unit ofgenerated power would also be of importance to ensure a reasonable
cost for a given performance.
In any case, thousands of microwave tubes or millions of solidstate amplifiers and oscillators have to be phased and controlled,which is a large technical challenge.

3.3 Rectennas
The rectenna (located on the Earth) receives the microwave powerfrom the SPS and converts it to dc electricity (e.g., [35]). The
rectenna is composed of an RF antenna, a low-pass filter, and a
rectifier. It is a purely passive system (apart from a low-power pilot
beam: see Section 3.4) and needs no extra power. A low-pass filter isnecessary to suppress the microwave radiation that is generated by
nonlinearities in the rectifier. Most rectifiers use Schottky diodes.
Various rectenna schemes have been proposed, and the maximum
conversion efficiencies anticipated so far are 91.4% at 2.45 GHz [36]and 82% at 5.8 GHz [37]. However, the actual rectenna efficiency
will also depend on various other factors, such as the microwave
input power intensity and the load impedance.
The single elements of the rectenna can be of many types, such as
dipoles, Yagi antennas, microstrip antennas, or even parabolic
dishes.
The rectenna array, with a typical radius of approximately 2 km, is
an important element of the radio technology for which highefficiency is essential. The efficiency depends on the input power,
and the input-power flux density is not constant over the entire
rectenna site for the SPS system. Further research will be required
into rectennas that maintain high efficiency under various inputpower conditions. Recently, development has started on a low-power(only 100 µW or less), high-efficiency rectenna system for the
perimeter of the rectenna site [38]. Studies and experiments have also
been performed for a hybrid technique [39].

18

3.4 Control and Calibration
Another important issue concerning the space-based microwave
antenna is the necessarily high precision of the control of the beam
direction. This is important for two reasons: to maximise the energy
transferred to the Earth; and to limit radiation in undesired directions,in order to avoid adverse effects on existing telecommunications,passive radio-detection systems, and biological systems. This goal
may be achieved with the concept of a retrodirective array, in which
the rectenna sends a pilot signal to the SPS in order to indicate its
position before the power beam is transmitted. This pilot beam isthen used to direct the power beam back along exactly the same pathas the pilot beam: in the retrodirective direction. The effect of this is
to automatically remove perturbations to the direction of the
propagating beam, assuming that the perturbing factors along the
propagation path do not change during the round-trip transit time.
For this to work, retrodirective beam-forming techniques have to be
developed in order to suppress sidelobes and to maximise thetransmission efficiency. In addition, control measures have to takethe delay of commands into account, which is a considerable fractionof a second for an SPS in geostationary orbit.
Emergency procedures should be defined and have to be applied
when the beam direction is not contained within the predefined angleof 0.0005°. Ordering an interruption of the RF transmission may be apossible solution, but the detrimental effects that could be caused bya sudden interruption of the dc-to-RF conversion onboard thesatellite have to be evaluated, not forgetting that the load to the grid
will also need to be managed carefully.
The centre of the microwave beam should be confined to a region
within 0.0005° of the centre of the rectenna. This corresponds to lessthan one-fourth of the 8-arc-seconds half-power beamwidth of a
1000-m-diameter parabolic SPS antenna. Achieving such pointing
accuracy and stability would currently pose a major technicalchallenge. The required beam-control accuracy of the SPS
microwave power-transmission system may be achieved using a verylarge number of power-transmitting antenna elements, and bylimiting the total phase errors over the antenna array to a few
degrees. Technologies to achieve these goals are presently under19

study [18]. Beam-collection efficiency is as important as the beamcontrol accuracy, and the efficiency depends on the power lost in
sidelobes and grating lobes.
Measurement and calibration are important issues for the SPS andmicrowave power-transmission systems, because the SPS’s
microwave power-transmission system requires accurate beam
control with a large phased array. The testing of large SPS antennas
presents not only the usual difficulty of making accurate RF
measurements over a substantial aperture, but also the unusual
problems of devising tests that can accurately predict theperformance of the antenna under the harsh mechanical, thermal, andradiation conditions in the space environment. New methods ofmeasurement and calibration would therefore need to be developed.
Microwave measurements and calibration would be necessary for theevaluation of power, interference, and spurious emissions from the
SPS and rectennas.
The proposed antennas – both the transmitting antenna and therectenna – are expected to be so large that testing them in their
entirety will pose significant challenges. Computer simulations cangive accurate predictions of the performance of the antennas in termsof gain, beamwidth, and near sidelobes. However, the transmitting
antenna can only be accurately tested once in orbit, and to achievethis, special antenna-measurement and calibration techniques willneed to be developed.

4. Radio Science Influences and Effects ofSPS
4.1 Interaction with the Ionosphere andAtmosphere
To a first approximation, it is generally considered that theinteraction of the SPS system with the medium – space, ionosphere,
and atmosphere – will be negligible. However, as noted in
Section 2.6, SPS subsystems may be affected by accelerated solarwind particles and solar radiation. Space is a harsh environment, with20

large temperature gradients and ionising radiation (geostationary
satellites are in the solar-wind regime during large geomagnetic
storms). On the other hand, currents created by SPS may locallyaffect the medium [40].
Power loss due to normal atmospheric absorption over the distancefrom a geostationary orbit to the ground is assumed to be below 2%.
In abnormal circumstances, significant departures might be expected
when, for instance, the beam encounters scintillations in theionosphere and rain cells in the troposphere, as explained in thefollowing.
Very few groups have worked on the effects of powerfulmicrowaves on the atmosphere and ionosphere, and the few studies
presently available refer to potential effects via the heating of
ionospheric electrons or via ionisation of the air. The expertise islimited, but it exists. However, at a time where new observations(transient luminous events, terrestrial gamma-ray flashes) raise newquestions about energy coupling between the atmosphere and thespace environment [41], studies are needed on all phenomena that
may influence the atmospheric electrical conductivity and chemicalcomposition.
In the process of SPS construction, large high-power electric
propulsion systems would be needed to move the structures from a
low Earth orbit to the geostationary orbit. These would inject heavy
ions perpendicular to the Earth’s magnetic field (around the equator).
The injection could strongly disturb the electromagnetic environment
surrounding the ion engine in the ionosphere and the magnetosphere,
through interaction between the heavy-ion beam and the ambient
plasmas. Some of these effects are discussed in [1, Section 4.1.3].
A thorough and systematic theoretical analysis of possible
ionospheric effects was published under an ESA contract [42]. This
analysis indicated several possible relevant effects, but
simultaneously stated that “the natural variability of the ionosphere,
as well as the fundamental unpredictability of nonlinear effects
certainly limit the accuracy with which the performance of SPS
systems and their environmental impact can be estimated.”
In principle, radio waves passing through the ionosphere areabsorbed due to ohmic heating, i.e., wave energy heats the electrons.
This effect is strongest in the ionospheric D and E layers, but theeffect is assumed to be small for radio-wave frequencies above21

1 GHz, since the heating efficiency varies as the inverse square of
the frequency. No ground-based measurements of electron heatingby high-power microwaves are available in the GHz range; only
theoretical estimates exist for a frequency of 3 GHz [43]. Theseestimates indicate that an electron-temperature increase from 200 K
to 1000 K in the E layer might occur for a power-flux density of
500 W/m2. Test microwave injections from a sounding rocket havebeen carried out in Japan [12]. Although ohmic-heating effects were
not observed, plasma waves were excited by the injected
microwaves. This was in agreement with several theoreticalpredictions that high-power microwaves may produce plasma
instabilities in the ionosphere (e.g., [42]). Several types of such
instabilities produce secondary electromagnetic waves, which could
be a source of interference to other radio services. The instabilities
might also result in additional electron heating and density
irregularities, which could have an effect on other radio waves
propagating through the region. It is uncertain if the SPS microwave
power-flux density would be high enough to cause such effects, orwhether these effects could affect the SPS microwave transmissions.
Another problem may be defocusing of the microwave power
beam, due to naturally occurring electron-density irregularities
causing rapid signal-strength fluctuations (scintillations). This could
have severe implications for the beam control described in
Section 3.4, but, again, it is not known if this effect is important for
the envisioned frequency of the SPS microwave beam. Theoreticalconsiderations show that a 2.45-GHz SPS system would be morestrongly affected than a 5.8-GHz system [42]. The effects ofdefocusing and scintillation on natural irregularities will be there forall power densities. What is uncertain is whether the high SPS power
densities would enhance the effect through nonlinear interactions andfeedback.
Some effects of powerful microwaves on the stratosphere havebeen studied both theoretically and experimentally [44]. These
investigations have been carried out for a quite different purpose,
namely to study the effects of ozone-destroying pollutants in the
troposphere, and to create an artificial ozone layer in the stratosphereby high-power electromagnetic waves. The field strength necessary
for this is much higher than the values that would be used by an SPS.
Therefore, such effects on the atmosphere are not expected.

22

In the troposphere, refraction and scintillation effects on the beam(or even those induced by the high-power beam itself) need to beconsidered. Also, absorption and diffraction by atmospheric gases,
aerosols, (water/ice) clouds, and precipitation must be studied. For
instance, Recommendation ITU-R P.619 states the following about
interference from an SPS: “Using available data on likely harmonic
content, it can be shown that – even at the 4th harmonic – theinterfering signal at a distance of 50 km from the rain cell can becomparable with the level of the received signal in the fixed satellite
service. At the fundamental frequency, however, direct radiationfrom the side lobes of the SPS to the terrestrial station will probably
exceed the signal due to precipitation scatter.” In addition, two othereffects have to be taken into account: beam attenuation and beamdiffusion due to rain. As an example, for a cloud temperature of 0° C
and a path length under rain of 4 km, the absorption at 5.8 GHz is
0.16 dB, 1.2 dB, and 2.8 dB for precipitation rates of 10 mm/h,
50 mm/h, and 100 mm/h, respectively [45]. Although rain rates of
100 mm/h are rare [1], it has to be stated that the last figure
corresponds to a power loss of almost 50%. The beam diffusion at a−30 dBW level can be as large as 4-6 km in diameter for
precipitation rates of 50-100 mm/h [45].

4.2 Compatibility with Other Radio Services
and Applications
It is assumed that typical SPS systems will use frequency bands
around 2.45 GHz or 5.8 GHz. These bands are already allocated in
the ITU-R Radio Regulations to a number of radio services (e.g.
civilian and military wireless applications), and are also designatedfor ISM (industry, science, and medical) and applications such as
microwave ovens and wireless LANs [46]. It is mandatory thatunwanted emissions – such as carrier noise, harmonics, and spurious
and out-of-band emissions of the microwave power-transmission
beams – are suppressed sufficiently to avoid interference with otherradio services and applications, in accordance with the ITU-R Radio
Regulations [46]. This is a serious engineering challenge, given thehuge disparity between SPS power levels and those of other radio
services. Although the intended bandwidth of the SPS emissions is23quite narrow – since an essentially monochromatic wave without
modulation will be used – spurious and out-of-band emissions
generated by microwave power-transmission beams could
substantially degrade the performance of other services andapplications, even if received only indirectly.
Of particular concern is interference with radio-astronomical
observations, which have a protected band (4.9-5.0 GHz) near theenvisioned SPS frequencies or their first harmonic. Radio astronomy
has historically increased its sensitivity with time, and in the next
decade, major initiatives already begun will enhance the sensitivity
by 100 fold over existing instruments. All possible measures need tobe taken to protect the corresponding observations, since if theycannot be protected, it would not be possible for an SPS to operate
legally under the present ITU regulations. Most experts will agree
that even a partially operational SPS will constitute a difficult andunwelcome challenge to radio astronomy, and that the coexistence ofradio-astronomical observations with an SPS could be extremely
difficult. The same applies for measurements by the Earthexploration satellite services (e.g., a sub-harmonic of 2.45 GHz isclose to 1.4 GHz, used for passive sensing of soil moisture and ocean
salinity). In 1997, the ITU initiated work towards an ITU-R
Recommendation on wireless power transmission [4], which may be
relevant to the interference an SPS could cause to other services.
The possibility of spurious emissions related to tube (e.g.
magnetron) failure is a serious concern for radio astronomy andmany other services. For example, with 10,000 magnetrons of
100 kW output for the microwave transmission, and assuming amean time to failure of, say, 30 years for these tubes, it is possiblethat the average failure rate could be one per day at some point in thelife cycle.
Furthermore, the passive thermal radiation of the solar cells of alarge number of SPS units is expected to make a substantial zone of
the sky, centred on the geostationary orbit, unusable for astronomical
observations at essentially all frequencies [47]. This would occur
even when the microwave transmission of the SPS towards the Earthwas not operational.
In addition to this thermal radiation, the huge solar-cell array
would act as a broadband antenna for all radio noise created within
the SPS (from switching, out-of-band contributions, etc.). Therefore,24such RF noise has to be minimised so as not to degrade operations ofradio services and applications.
The apparent angular size of a solar-cell array of 10 km2 is close to
1 arc minute (somewhat larger than the angular size of Jupiter), andscattering of unwanted radiation in the atmosphere would
substantially extend the affected region. This means that even opticalastronomy would be affected in an extended region of the sky,
particularly if a large number of SPS units were operational. The
substantial loss of observable sky resulting from these wideband
emissions (optical, UV, infrared, and radio) needs to be carefully
considered.
The requirements of spectral purity (a narrowband signal withvery low spurious transmission) and the high efficiency of thetransmitter will be opposing constraints. They could be difficult toreconcile, since high-efficiency, high-power transmitters have an
inherent problem of non-linearity. This needs to be carefully
assessed.
Astronomical Radio Quiet Zones (RQZs) are currently in theprocess of being implemented in isolated areas in, e.g., Australia,
China, and South Africa. This is being done to ensure the regulatory
protection of next-generation giant radio telescopes against
detrimental manmade radio interference over wide frequency ranges,
based on interference threshold levels recommended by the ITU.
Currently, regulatory control over the RQZs applies only to groundbased transmissions. However, for the zones to be effective, it isimportant that they are not exposed to harmful levels of emissions
from space. Even when an SPS is operating entirely within its
permitted frequency range, with no out-of-band transmissions, thepower transmitted within its sidelobes may still be harmful to theoperation of broadband radio telescopes in RQZs (and elsewhere).
An additional challenge will therefore be to devise solutions toprevent unwanted interference from the SPS into such facilities.
These solutions may include aspects of antenna design, location of
the SPS, and deployment of mitigation techniques at the radioastronomy sites.

4.3 Microwave Power Transmission Effects
on Human Health25

A variety of environmental considerations and safety-related
factors should continue to receive consideration because of publicconcerns about radiowave exposure [48]. Above the centre of therectenna, the SPS power-flux density will be considerably higher
than the currently permissible safety levels for human beings. The
ICNIRP (International Commission on Non-Ionising Radiation
Protection) and Japan both apply limits of 50 W/m2 and 10 W/m2 for2.45 GHz and 5.8 GHz, respectively [49]. The latter level is equal tothe power-flux density at the perimeter of the rectenna site [1,Section 4.3]. The corresponding exposure limits for IEEE standards
have recently been revised, and they are now closer to the ICNIRP
limits (see [50] for details).
Since established safety limits for microwave exposure areexceeded in an area around and above the rectenna during normal
operation of the SPS, access would need to be carefully controlled toensure that environmental safety and health standards aremaintained. Under normal operating conditions, the SPS microwave
downlink will need to be monitored continuously to ensure that thetightly tuned phased-array techniques and beam control arefunctioning correctly. Should there be a loss of control, beamdefocusing techniques to disperse the power would need to be
applied.
It should be noted that there are currently insufficient data on
specific microwave power-transmission effects on human health, andthat standards for this particular application are not sufficiently
developed. Taking into consideration the importance of this field,
more studies are urgently needed regarding human health and its
bioeffects (see also more details in [1, Section 4.3]).

5. Radio-Science Issues for Further Studies
The list of issues below is most likely not complete. Depending on
the outcome of the questions addressed, other issues may come up.
Again, the list is limited to issues of URSI’s scientific domain.

26

• Can the exposure level of the microwave density at the perimeter of
the SPS receiving rectenna site be adequately controlled to avoid
exceeding the safety level fixed by international standards?
• What is the impact of rectenna operation on (i) biological systems,
such as human beings, birds, insects, and plants, etc.; (ii) airborne
vehicles, such as airplanes; and (iii) other electric/electronicequipment and telecommunication networks?
• Can SPS operations be made safe by a precise control of the highpower beam using a pilot signal from the Earth, also taking into
account the time delay of the signal?
• The influences of atmospheric refraction, beam defocusing, and ofabsorption and diffraction by atmospheric gases, aerosols, clouds,
and precipitation have to be further examined. Are there othereffects caused by the SPS power beam on the environment(magnetosphere, ionosphere, troposphere, etc.) that have not yet
been explored?
• What is the impact of SPS electromagnetic emissions – bothintended and unwanted (harmonics of the microwave frequency,
unexpected and harmful radiation resulting from malfunctions) at
microwave frequencies and other related frequencies – on
telecommunications, remote sensing, navigation satellite systems,
and radio-astronomical observations? What actions can be taken to
suppress this unwanted emission? Constraints imposed by theRadio Regulations of the International Telecommunication Union
must be taken into account.
• How will reflections of sunlight from the huge satellite structure
affect optical-astronomical observations, and how will passive
thermal emissions affect radio-astronomical observations?
• What potential is there for damage to the SPS system from space
weather?
• What are the consequences of long-term exposure to solar-windparticles, and solar radiation of solar cells and other solid-state27

devices, for the reliability and costs of SPS systems, taking
maintenance and possible replacement into account?
• Will an SPS lead to congestion at the geostationary orbit and tointerference with communication satellites?
Even if it is beyond URSI’s scientific domain, the economics of
SPS systems have to be examined by competent organisations, since
the cost advantage is a crucial issue for the feasibility of the whole
SPS concept.
Only some parts of these questions can be addressed by laboratory
work, simulations, or system analyses. Tests of the large structures(solar-cell arrays, transmitting antenna, mirrors) in space aremandatory. After successful testing, launching a pilot SPS unit as an
operational demonstrator project – presumably with broadinternational consensus – may be a suitable way to assess theremaining questions. However, before being considered for launch,even for such a pilot unit, all concerns, such as the impact oncommunications, radio astronomy, Earth observations, and biohazards, must be fully addressed.

6. Acknowledgements
This white paper is based on detailed reports on SPS systems
prepared by the URSI Inter-Commission Working Group on SPS(SPSICWG) [1]. The URSI Board of Officers is indebted to themembers of this Working Group and to R. M. Dickinson, D. Farley,
R. Gendrin, and L. Summerer for their help in evaluating this paper.

28

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http://www.ursi.org.
2. S. Fetter, “Space Solar Power: An Idea Whose Time Will Never
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3. A. Smith, “Earth vs. Space for Solar Energy, Round 2,” Physics
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5. P. Glaser, “Power from the Sun: Its Future,” Science, 162, 22
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6a. W. C. Brown, “Satellite Power Stations: A New Source ofEnergy,” IEEE Spectrum, 10, 3, 1973, pp. 38-47.
6b. W. C. Brown, “The History of Power Transmission by Radio
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9. H. Feingold, et al., “Evaluation of Comparison of Space Solar
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11. L. Summerer, “Solar Power from Space – European Strategy inthe Light of Global Sustainable Development,” ESA SPS
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12. H. Matsumoto, N. Kaya, I. Kimura, S. Miyatake, M. Nagatomo,
and T. Obayashi, “MINIX Project Toward the Solar Power
Satellite – Rocket Experiment of Microwave Energy29Transmission and Associated Nonlinear Plasma Physics in the
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13. N. Kaya, H. Matsumoto, and R. Akiba, “Rocket Experiment
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11, 3-4, 1992, pp. 267-274.
14. M. Shimokura, N. Kaya, N. Shinohara, and H. Matsumoito,“Point-to-Point Microwave Power Transmission Experiment,
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Cyclotron Waves by a Monochromatic Strong Microwave:
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16. H. Matsumoto, “Numerical Estimation of SPS Microwave Impact
on Ionospheric Environment,” Acta Astronautica, 9, 1982, pp.
493-497.
17. M. Nagatomo and K. Itoh, “An Evolutionary Satellite Power
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18. N. Shinohara, Y. Hisada, M. Mort, and JAXA SSPS WG4 Team,“Request and Roadmap for Microwave Power Transmission
System of Space Solar Power System (SSPS),” Proc. of IAF2005,
Japan, 2005.
19. Y. Kobayashi, T. Saito, K. Ijichi, and H. Kanai, Proc. of the 4th
Int. Conf. on Solar Power from Space – SPS ‘04, July 2004,
Granada, Spain, ESA SP-567, December 2004.
20. “Special Sections on SSPS,” Radio Science Bulletin, Nos. 310
and 311, 2004.
21. Proc. of the 4th Int. Conf. on Solar Power from Space – SPS ‘04,
July 2004, Granada, Spain, ESA SP-567, December 2004.
22. R. M. Dickinson and W. C. Brown, “Radiated Microwave Power
Transmission System Efficiency,” NASA TM 33-727, JPL, CIT,
Pasadena, CA, May 15, 1975.
23. National Research Council, Laying the Foundation for Space
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Investment Strategy, Washington, DC, USA, National Academy
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24. NASA – Marshall Space Flight Center, Press Release 98-190:
http://www.msfc.nasa.gov/news/news/releases/1998/98-190.html.
25. H. Cikanek, “Innovative Aerospace Propulsion Systems an
Technologies,” NASA Glenn Research Center, 2-4 April 2000,
Report No. 216-433-6196, http://www.aero-space.nasa.gov/
events/home&home/glenn/invasp/sld003.htm.
26. ESA General Studies Programme, “System Concepts,
Architectures and Technologies for Space Exploration andUtilisation (SE&U Study), Executive Summary,” Contract
127/98/NL/JG(SC),http://www.esa.int/SPECIALS/GSP/SEMTG7
0P4HD_0.html.
27. http://rredc.nrel.gov/solar/old_data/nsrdb/redbook/atlas/serve.cgi.
28. V. Blandow, P. Schmidt, W. Weindorf, M. Zerta, and W. Zittel,“Earth and Space-Based Power Generation Systems – A
Comparison Study,” Final Report 17682/03/NL/EC, ESA final
report – LBST, 2004.
29. L. Summerer, M. Vasile, R. Biesbroek, and F. Ongaro, “Spaceand Ground Based Large Scale Solar Power Plants – European
Perspective,” IAC-03/R.1.09, 2003.
30. M. Imaizumi, K. Tanaka, S. Kawakita, T. Sumita, H., Naito, andS. Kuwajima, “Study on Power Generation System for a Space
Photovoltaic Power Satellite,” Proceedings of 48th Space
Sciences and Technology Conference, 2004, pp. 111-115.
31. G. A. Landis, “Reinventing the Solar Power Satellite,”
NASA/TM-2004-212743, 2004, pp. 1-30.
32. S. Kawasaki, “A Unit Plate of a Thin Multilayered Active
Integrated Antenna for Space Solar Power System, Radio Science
Bulletin, No. 310, 2004, pp. 15-22.
33. N. Shinohara, H. Matsumoto, and K. Hashimoto, “Solar Power
Station/Satellite (SPS) with Phase Controlled Magnetrons,”
IEICE Trans. Electron., E86-C, 2003, pp. 1550-1555.
34. K. Nanokaichi, N. Shinohara, S. Kawasaki, T. Mitani and H.
Matsumoto, “Development of Waveguide-Slot-Fed Active
Integrated Antenna for Microwave Power Transmission,”
Proceedings of the XXVIIIth General Assembly of International
Union of Radio Science (URSI), New Delhi, India, October 23-29,
2005, D08.4 (0950).

31

35. J. Zbitou, M. Latrach, and S. Toutain, “Hybrid Rectenna andMonolithic Integrated Zero-Bias Microwave Rectifier,” IEEE
Transactions on Microwave Theory and Techniques, 54, 2006,
pp. 147-152.
36. W. C. Brown, “Electronic and Mechanical Improvement of theReceiving Terminal of a Free-Space Microwave Power
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Report PT-4964, NASA Report No. CR-135194, NASA Contract
No. NAS 3-19722, August 1977, p. 66.
37. J. O. McSpadden, L. Fan, and K. Chang, “Design andExperiments of a High-Conversion-Efficiency 5.8-GHz
Rectenna,” IEEE Transactions on Microwave Theory andTechniques, MTT-46, 12, December 1998, pp. 2053-2060.
38. N. Shinohara, H. Matsumoto, A. Yamamoto, H. Okegawa, T.
Mizuno, H. Uematsu, H. Ikematsu, and I. Mikami, “Developmentof High Efficiency Rectenna at mW input,” Technical Report of
IEICE, SPS2004-08 (2005-04), pp. 15-20 (in Japanese).
39. N. Shinohara and H. Matsumoto, “Dependence of dc Output of aRectenna Array on the Method of Interconnection of its Array
Elements,” Electrical Engineering in Japan, 125, 1, 1998, pp. 917.
40. J. F. Drake, M. Swisdak, H. Che, and M. A. Shay, “Electron
Acceleration from Contracting Magnetic Islands During
Reconnection,” Nature, October 5, 2006.
41. S. A. Cummer, Y. Zhai, W. Hu, D. M. Smith, L. I. Lopez, and M.
A. Stanley, “Measurements and Implications of the Relationship
Between Lightning and Terrestrial Gamma Ray Flashes,”
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42. T. R. Robinson, T. K. Yeoman, and R. S. Dhillon,“Environmental Impact of High Power Density Microwave
Beams on Different Atmospheric Layers,” Radio and Space
Plasma Physics Group Tech. Rep. 63, ESA Contract number:
18156/04/NL/MV, Leicester University, UK, 2004.
43. F. W. Perkins and R. G. Roble, “Ionospheric Heating by Radio
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44. G. M. Batanov, I. A. Kossyi, and V. P. Silakov, Plasma Physics
Reports, 28, 3, 2002, pp. 204-228 (translated from Fizika Plazmy,
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46. International Telecommunication Union, Radio Regulations,edition of 2004.
47. A. R. Thompson, “Effects of a Satellite Power System on
Ground-Based Radio and Radar Astronomy,” Radio Science, 16,1981, pp. 35-45.
48. J. C. Lin, “Space Solar-Power Stations, Wireless Power
Transmissions, and Biological Implications,” IEEE Microwave
Magazine, March 2002, pp. 36-42.
49. ICNIRP, “Guidelines for Limiting Exposure to Time-Varying
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33

8. Appendix 1: URSI White Papers
URSI (International Union of Radio Science) white papers are
documents issued by URSI scientific experts on controversial
subjects involving aspects of radio science. They may be proposed to
the URSI Board of Officers by an URSI Member Committee, an
URSI Commission, an URSI Standing Committee, an URSI Working
Group, or in response to a request to URSI by another body.
Where the issuance of a white paper is determined to be necessary,
the appropriate mechanism for preparing it is agreed to between the
URSI Secretariat and the URSI Board of Officers. Once a draft URSI
white paper has been prepared, the URSI Secretariat forwards this tothe members of the Board of Officers and to the URSI Member
Committees and Commissions for review. All comments received by
the URSI Secretariat are then either incorporated directly into thewhite paper, if appropriate, or are forwarded to the author(s) for
consideration. The URSI Secretariat acts as a liaison throughout thisprocess. The final version of the URSI white paper is then sent to themembers of the URSI Board of Officers for approval. Finally, thewhite paper is distributed to ICSU, appropriate scientific unions andbodies, and is published in the Radio Science Bulletin.
The white paper is the responsibility of URSI. However, it does
not necessarily reflect all the views of the individual URSI Member
Committees nor of the Commissions.

34

9. Appendix 2: the ten ScientificCommissions of URSI and their Terms ofReference
9.1 Commission A: Electromagnetic Metrology,
Electromagnetic Measurements, and Standards
The Commission promotes research and development in the fieldof measurement standards, in calibration and measurement
methodologies, and the intercomparison of such. Areas of emphasisare:(a) the development and refinement of new measurement
techniques;(b) primary standards, including those based on quantum
phenomena;(c) realization and dissemination of time and frequency standards;(d) characterization of the electromagnetic properties of materials;(e) electromagnetic dosimetry.
The Commission fosters accurate and consistent measurements
needed to support research, development, and exploitation ofelectromagnetic technologies across the spectrum.
9.2 Commission B: Fields and Waves, Electromagnetic
Theory and Applications
The interest of Commission B is fields and waves, encompassing
theory, analysis, computation, experiments, validation, andapplications. Areas of emphasis are:(a) Time-domain and frequency-domain phenomena;(b) Scattering and diffraction;(c) General propagation including waves in specialised media;(d) Guided waves;(e) Antennas and radiation;(f) Inverse scattering and imaging.
The Commission fosters the creation, development, and refinementof analytical, numerical, and measurement techniques to understandthese phenomena. It encourages innovation and seeks to applyinterdisciplinary concepts and methods.

35

9.3 Commission C: Radio-Communication Systems andSignal Processing
The Commission promotes research and development in:(a) Radio-communication and telecommunication systems;(b) Spectrum and medium utilisation;(c) Information theory, coding, modulation, and detection;(d) Signal and image processing in the area of radio science.
The design of effective radio-communication systems must include
scientific, engineering, and economic considerations. This
Commission emphasises research into the scientific aspects, andprovides enabling technologies to other areas of radio science.
9.4 Commission D: Electronics and Photonics
The Commission promotes research and reviews new
developments in:(a) Electronic devices, circuits, systems, and applications;(b) Photonic devices, systems, and applications;(c) Physics, materials, CAD, technology, and reliability ofelectronic and photonic devices down to nanoscale including
quantum devices, with particular reference to radio science andtelecommunications.
The Commission deals with devices for generation, detection,
storage, and processing of electromagnetic signals together with theirapplications from the low frequencies to the optical domain.
9.5 Commission E: Electromagnetic Noise andInterference
The Commission promotes research and development in:(a) Terrestrial and planetary noise of natural origin, seismicassociated electromagnetic fields;(b) Man-made noise;(c) The composite noise environment;(d) The effects of noise on system performance;(e) The lasting effects of natural and intentional emissions onequipment performance;(f) The scientific basis of noise and interference control,
electromagnetic compatibility;(g) Spectrum management.

36

9.6 Commission F: Wave Propagation and Remote
Sensing (Planetary Atmospheres, Surfaces, andSubsurfaces)
The Commission encourages:(a) The study of all frequencies in a non-ionised environment:(i) wave propagation through planetary, neutral atmospheres,and surfaces(ii) wave interaction with the planetary surfaces (including land,
ocean, and ice), and subsurfaces,(iii) characterisation of the environment as it affects wave
phenomena;(b) The application of the results of these studies, particularly in theareas of remote sensing andcommunications;(c) The appropriate co-operation with other URSI Commissions
and other relevant organisations.
9.7 Commission G: Ionospheric Radio and Propagation(Including Ionospheric Communications and Remote
Sensing of Ionised Media)
The Commission deals with the study of the ionosphere in order toprovide the broad understanding necessary to support space andground-based radio systems. Specifically, the Commission addresses
the following areas:(a) Global morphology and modelling of the ionosphere;(b) Ionospheric space-time variations;(c) Development of tools and networks needed to measureionospheric properties and trends;(d) Theory and practice of radio propagation via the ionosphere;(e) Application of ionospheric information to radio systems.
To achieve these objectives, the Commission co-operates with other
URSI Commissions, corresponding bodies of the ICSU family(IUGG, IAU, COSPAR, SCOSTEP, etc.) and other organisations(ITU, IEEE, etc.).
9.8 Commission H: Waves in Plasmas (Including Spaceand Laboratory Plasmas)
The goals of the Commission are:37

(a) To study waves in plasmas in the broadest sense, and inparticular:(i) the generation (i.e. plasma instabilities) and propagation ofwaves in plasmas,(ii) the interaction between these waves, and wave-particle
interactions,(iii) plasma turbulence and chaos,(iv) spacecraft-plasma interaction ;(b) To encourage the application of these studies, particularly tosolar/planetary plasma interactions, space weather, and the
exploitation of space as a research laboratory.
9.9 Commission J: Radio Astronomy (including Remote
Sensing of Celestial Objects)(a) The activities of the Commission are concerned withobservation and interpretation of all radio
emissionsandreflections from celestial objects.
(b) Emphasis is placed on:(i) the promotion of technical means for making radioastronomical observations and data analysis,(ii) support of activities to protect radio-astronomical
observations from harmful interference.
9.10 Commission K: Electromagnetics in Biology andMedicine
The Commission is charged with promoting research anddevelopment in the following domains:(a) Physical interaction of electromagnetic fields* with biologicalsystems;(b) Biological effects of electromagnetic fields;(c) Mechanisms underlying the effects of electromagnetic fields;(d) Experimental electromagnetic fields exposure systems;(e) Assessment of human exposure to electromagnetic fields;(f) Medical applications of electromagnetic fields.
* (frequency range from static to terahertz)
More information about URSI can be obtained from its Web pages
at http://www.ursi.org.

38

Report of the URSI Inter-Commission
Working Group on SPS

Chapter 1

Background of SPS Researchand Developments

1.1 Scope of human life in the coming hundred
years
Mankind has recently enhanced its living standard and its
population in an explosive way. In fact, the human population
quadrupled and primary power consumption increased 16-fold 1during the 20th century. The consumption of energy, food, andmaterial resources is predicted to increase 2.5 fold in the coming 50
years. As a result of our efforts for better life, we have come to face,in this 21st century, serious global issues threatening our safe life oreven our existence itself on our mother planet Earth. These are issues
such as global warming, environmental degradation, declining
nutrition on land and sea from rising CO2, and rapid decrease offossil reservoir. Since the living standard and the population ofdeveloping countries are increasing continuously, the demand ofenergy will be several times larger than that of today's requirement
by 2050.

1.2 Energy demands in the next 50 years
One primary power source at present comes from fossil fuels suchas oil, coal and natural gas. However, the fossil fuels have twoserious factors that prevent them from being used for as a long-term
primary power source. One is their limited amount; they will not lastlong if used at the same or higher pace than that of today. The otheris that they emit carbon dioxide, a green house gas, which causes
global warming.
1.2.1
Predictions of demand and production of fossil fuels
On Nov. 9, 2004, Forbes reported that Russian oil exports may
decrease within two years.2 “Further growth is possible only if pricetrends are good,” a Russian expert said. Arabicnews.com reported
that a sharp decrease in Syria's light crude oil exports is expected3
on Nov. 17. Such decrease of oil production is not surprising. M. K.
Hubbert predicted in 1956 that crude oil production from U.S.
(except Alaska) would crest in 1969. Figure 1.2.1 depicts the annual
oil productions. The lighter lines are predictions according to39

Campbell and Laherrere’s model, based in part on multiple Hubbert
curves.5 US and Canadian oil indicated by the brown line peaked in1972 as predicted by Hubbert. Global annual oil production shown in
the red line, recovered after falling in 1973 and 1979, but a morepermanent decline is seen in recent years. Production in the former
Soviet Union (yellow) has fallen 45 percent since 1987. A crest in theoil produced outside the Persian Gulf region (purple) now appears
imminent. Figure1.2.2 illustrates recent trends of the production that
supports the prediction. The cost of extracting the next barrel of oilor cubic meter of natural gas is continually escalating, threatening to
ignite major and global inflationary pressures due to rising energy
costs. Figure 1.2.3 indicates that the industry will likely need to add
100 million oil-equivalent barrels per day, which is close to 80% of
today’s production level, by 2015 to meet the demand according to
an Exxon report.7
In 2000, the world had 6.1 billion human inhabitants. This number
could rise to more than 9 billions in the next 50 years as shown in
Fig. 1.2.4. This future population increase is mostly due to very rapid
increase in less developed countries although the number in moredeveloped countries will be almost constant (about 1 billion) or evendecrease.4

Fig. 1.2.1 Global production of oil. Lighter lines are
predictions.5
40

Fig. 1.2.2 Oil and gas liquids 2004 scenario from 1930 to 2050.6

Fig. 1.2.3 Supplying oil and gas demand.7
41

Fig. 1.2.4 World population prospect 7

Fig. 1.2.5 Predicted absolutecivilization in 2050.8

shortage of resources for humanIn the last century, despite two world wars, mankind experienced
an explosive increase of both its Standard of Living (SoL) and its
population (see Fig. 1.2.5). The explosive increase in the humanpopulation inevitably requires an exponential increase in the42consumption of energy, food, and material resources. This fact has
led us into today’s global issues such as global warming,
environmental change, and rapid decrease of the fossil reservoir.8
Matsumoto represented the consumption in an equivalent weight,
SoL tons. This will not increase in developed countries, but will
increase in developing countries due to their better living. Combining
this with the population increase, the absolute shortage of resources
becomes clear.
Figure 1.2.5 schematically depicts a simple calculation of the total
resources required to maintain human civilization in 2050.8 The
upper half panel presents the consumption of resources for living
today in relative units of “SoL tons.” The population in thedeveloped countries is approximately 1 billion, and that in thedeveloping countries is 5 billions. The SoL in the developed
countries is ten times higher than that in the developing countries. In
the developing countries the total resources consumed by humans
today including energy, food and materials for daily and industrial
demands, are 15 billion SoL-ton persons. By 2050, the population in
the developing countries will reach 9 billion and their SoL will be atleast three times higher than now. This increases the total resources
needed to maintain the world economy and welfare of daily humanlife to 37 billion SoL-ton persons. This is more than 250% of today’s
requirements, and is not likely to be reached without the destructionof the environment of our mother planet Earth. It is highly probablethat the demand of electrical power, 16,661 TWh in 2003, will
increase at a much higher pace than other energy demands as theworld becomes more industrialized and computerized.
1.2.2
CO2 emissions from fossil fuel use9
Arrhenius predicted in the 19th century that CO2 from fossil fuelburning could raise the infrared opacity of the atmosphere enough towarm the Earth.10 The fossil fuel greenhouse theory has become
more credible as observations accumulate and as we better
understand the links between fossil fuel burning, climate change, andenvironmental impacts.11 Atmospheric CO2 has increased from 275
parts per million (ppm) before the industrial era began to 379 ppm inMarch 2004 as shown in Fig. 1.2.6. Some scientists suggest that it
will pass 550 ppm this century. Climate models and paleoclimate
data indicate that 550 ppm, if sustained, could eventually produce
43

global warming comparable in magnitude but opposite in sign to theglobal cooling of the last Ice Age.12 This 550 ppm (strictly speaking,ppmv: ppm by volume) was used as the most frequently used
mitigation target.13

Fig. 1.2.6 Atmospheric carbon dioxide monthly mean mixing ratios.
Data prior to May 1974 are from the Scripps Institution of
Oceanography (SIO, blue), data since May 1974 are from the
National Oceanic and Atmospheric Administration (NOAA, red). Along term trend curve is fitted to the monthly mean values.14

44

Fig. 1.2.7 CO2 concentration and temperature change based on
SRES scenarios.11
The future scenarios were published in the Intergovernmental
Panel on Climate Change (IPCC) Special Report on Emission
Scenarios (SRES).15 They are composed of four (A1, A2, B1, andB2) families and are summarized as follows. The A1 family
describes a future world of very rapid economic growth, global
population that peaks in mid-century and declines thereafter, and therapid introduction of new and more efficient technologies. This45

family develops into three groups: fossil intensive (A1FI), non-fossil
energy sources (A1T), or a balance across all sources (A1B). The A2
family describes a very heterogeneous world. Economic
development is primarily regionally oriented and per capita
economic growth and technological change are more fragmented andslower than in other storylines. The B1 family describes a convergent
world with the same global population as in the A1 storyline, but theemphasis is on global solutions to economic, social, andenvironmental sustainability, including improved equity, but without
additional climate initiatives. The B2 family describes a world inwhich the emphasis is on local solutions to economic, social, andenvironmental sustainability. It is a world with continuously
increasing global population at a rate lower than A2, intermediatelevels of economic development, and less rapid and more diverse
technological change than in the B1 and A1 storylines. While thescenario is also oriented toward environmental protection and social
equity, it focuses on local and regional levels. Anthropogenic CO2
concentrations resulting from the six scenarios are shown in Fig.
1.2.7, where IS92 is a scenario published in 1992 by IPCC. The
fossil intensive A1FI group is worst and the sustainable B1 family is
best.
Figure 1.2.8 plots atmospheric CO2 Concentrations, in ppmv (parts
per million by volume) and global mean temperature, historicaldevelopment in various scenarios to 2100 worked by WEC (worldenergy council) and IIASA (International Institute of Applied
Systems Analysis).16 This was published between IS92 and SRES
scenarios. Case A family assumes high growth: A1 emphasizes oiland natural gas use; A2 coal-intensive; and A3 emphasizes the rolesof natural gas, renewables, and nuclear). Case B is a reference. Case
C is ecologically driven and has the lowest energy consumption andgreenhouse gas emissions. Case C family is divided into C1, whichassumes energy efficiency improvements, new renewables like solar,
and termination of nuclear by 2100, and C2, which assumes nuclear
power. The WEC believes curve C to be achievable, but only if weare really serious about being more careful and efficient in our use ofenergy, and also if we stimulate rather rapidly the growth of energy
generation from renewable resources.

46

Fig. 1.2.8. Atmospheric CO2 concentrations, in ppmv, historicaldevelopment from 1950 to 1990 and in scenarios to 2100. The insert
shows global mean temperature change compared with 1990 indegrees Celsius. The (substantial) model uncertainties are also
indicated.16

1.3 Kyoto Protocol and global warming
The Conference of Parties III (COP3), the Kyoto conference on
climate change, was held in Kyoto, Japan, in 1997. In order to solve
the crisis of the global warming, many countries agreed to specifictargets for cutting emissions of greenhouse gases from developed
countries at least 5% below 1990 levels during the period from 2008
to 2012. This agreement is called the Kyoto Protocol to the United
Nations Framework Convention on Climate Change (UNFCCC). It
was opened for signature on March 16, 1998, and closed on March
15, 1999. The Kyoto Protocol came into force on February 16,
2005 following ratification by Russia on November 18, 2004.
Nuclear energy is, apart from the difficulties of handling the waste,
not regarded as a renewable energy source.
Stabilizing the carbon dioxide–induced component of climatechange is an energy problem. Establishment of a course toward such47

stabilization will require the development within the coming decadesof primary energy sources that do not emit carbon dioxide to theatmosphere, in addition to efforts to reduce end-use energy demand.
Table 1.3.1 CO2 emission targets assigned to categorized countries
required by Kyoto Protocol17
Target(1990**Country
2008/2012)
EU-15*, Bulgaria, Czech Republic, Estonia,
Latvia, Liechtenstein, Lithuania, Monaco,-8%
Romania, Slovakia, Slovenia, Switzerland
US***
-7%
Canada, Hungary, Japan, Poland-6%
Croatia-5%
New Zealand, Russian Federation, Ukraine0%
Norway+1%
Australia+8%
Iceland+10%* The EU’s 15 member States will redistribute their targets
among themselves, taking advantage of a scheme under the Protocol
known as a “bubble.” The EU has already reached agreement on how
its targets will be redistributed.
** Some EITs18 have a baseline other than 1990.
*** The US has indicated its intention not to ratify the Kyoto
Protocol.

1.4 Sustainable energy Sources
In spite of environmental issues and depletion of their resources, it
is an undeniable fact that modern society heavily relies on the fossil
fuels. According to International Energy Agency, fossil fuels provide
about 80% of the total primary energy supply, as depicted in Fig.
1.4.1.19 To ensure a safe life for our children, we need to establish
science and technology for a sustainable society. Such science andtechnology can be called Green Science and Technology (GST).
Technology for stabilization of the carbon dioxide emissions is oneof the key elements of the GST and requires development of primaryenergy sources that do not emit carbon dioxide to the atmosphere orthat are renewable. Such sustainable energy technologies include48

terrestrial solar energy, hydropower energy, wind energy, and otherenergy systems based on natural resources.

Fig. 1.4.1Fuel shares of world total primary energy supply in 200319
1.4.1
Terrestrial Solar Energy
Solar energy is clean and exhaustless, and the amount of the solar
energy falling to the Earth per hour is nearly twice as much as theannual energy usage consumed by all mankind.
Photovoltaic (PV) cells, which are made of semiconductor devices,
have already been put to practical use all the way from small goods
such as watches and calculators to terrestrial solar power plants.
Research and development of PV cells are conducted actively for the
purposes of increasing conversion efficiency, reducing production
cost, etc.
Solar thermal energy, which has been in widespread use for solar
water heaters, is another utilization of terrestrial solar energy.
Research and development of solar thermal power plants with opticalconcentration by mirrors are ongoing in some countries, whereas
solar thermal power plants have not been put into commercial service
due to economic and location conditions.
Basic problems of terrestrial solar energy are atmosphericattenuation, daily and seasonal variation, and affects by climate
conditions. The average solar energy density in space near the Earthis 1.37 kW/m2. Atmospheric absorption reduces this to about 1
kW/m2 at the surface of the earth on a sunny day. Solar energy
weakens on cloudy and rainy days, and of course it can not be49

acquired at night. Maintenance of solar cell panels or solar collectors
is also an important issue since contamination and dust can degrade
production of electricity.
1.4.2
Hydropower Energy
Hydropower is an exhaustless and renewable energy that isconverted into electricity by water flowing through turbines. It
provided 2.2% of the world primary energy supply in 2003,
according to International Energy Agency.19 Its capacity per plantvaried from more than hundreds of MW to less than hundreds of kW.
The output power per unit weight is 1,000 times greater than that ofwind generation. Hydropower is CO2 emission-free energy, whereas
impacts on the natural aquatic environment, such as water pollution,and injury of aquatic animals passing through turbines are considered
as environmental issues.
1.4.3
Wind Energy
Wind energy, which has been in widespread use for windmills, isclean, exhaustless and renewable, but not constant and is greatly
affected by natural conditions. Due to much research, the cost ofwind energy has dropped drastically (by 85%) during the last 20
years according to the US Department of Energy,20 and wind power
generators are operated as small-sized power plants in many
countries. The worldwide wind energy capacity has been steadily
increasing from 7.470MW in 1997 to 47.616MW in 2004, and is
expected to be reach 100MW in 2008, according to a press releasefrom World Wind Energy Association.21
1.4.4
Biomass
Biomass is renewable energy produced by biological resources
such as natural botanical resources, agricultural crops, and animal
waste. Biomass can be converted to various materials including
bio-fuels, chemical materials, and electricity. Bio-fuels frombotanical resources can balance out the CO2 emission when burned,
since they have already absorbed CO2 by photosynthesis. Handlingtechnologies, collection logistics, and infrastructure are important
aspects of the biomass resource supply chain, according to the US
Department of Energy.20
50

1.4.5
GeothermalThis renewable energy is widely and abundantly distributed in
volcanic countries. It has advantages over other sustainable energy
sources from the viewpoints of the steady supply and environmental
burdens. However, current geothermal power plants are an
exhaustible resource since their resources are from shallow ground.
1.4.6
HydrogenHydrogen is a plentiful resource produced from water orhydrocarbons, and the energy is clean and renewable sinceenergy-yielding reaction produces water when it is converted toelectricity. Hydrogen is also expected to be an energy transporter ortransportation fuel. A top priority issue of hydrogen energy is toassure the safety against explosion.
1.4.7
Ocean Thermal
The energy is produced by temperature difference between hot
water of ocean surface and cold water at a depth of several hundred
meters. The validation phase of the pilot power plant is almost
finished; nevertheless a commercial power plant is quite difficult to
put into service due to economic matters.
1.4.8
Tidal PowerThe clean, exhaustless, plentiful and renewable energy is obtained
from the difference in tide levels, but daily variations of the energy
are unavoidable. The largest power plant, the La Rance station in
France, generates 240MW.20
1.4.9
Wave
Wave energy is clean, exhaustless, plentiful and renewable, and its
potential of generation in the world is more than 2000 TWh/year.22
Research has been conducted in many countries, especially in Europe.
According to the ATLAS Project, many of the current uncertainties
on cost and performance will need to overcome, although wave
energy is nearing the end of its R&D phase.23

1.5 Solar Power Satellite (SPS) as a Sustainable
Power Source
In contrast to the existing renewable power sources, Solar Power
51

from Space is promising in its 24-hour power supply capability andCO2-clean nature as a new energy system that can ensure sustainable
development of humanity. SPS was proposed several decades ago asa feasible candidate to satisfy the demand of sustainable andCO2-clean power supply usable as base load.
US National Research Council evaluated the SPS activities called
the SERT program as feasible and stated as follows in Summary in
2001.24
The committee has examined the SERT program’s technicalinvestment strategy and finds that while the technical andeconomic challenges of providing space solar power forcommercially competitive terrestrial electric power will
require breakthrough advances in a number of technologies,
the SERT program has provided a credible plan for making
progress toward this goal.
Descriptions of the SPS, its concept, and related technologies
especially in the field of URSI as well as scientific assessments andpossible impact of the SPS systems are presented in the following
chapters of this Report.

1.6 Nuclear Energy
Nuclear energy is COx and NOx emission-free, but its relative
share in the world primary energy supply in 2003 was only 6.5%
according to the International Energy Agency.19
The most important and serious issues of nuclear power
generations are proliferation and radioactive waste. Nuclear
accidents make major impacts on nuclear energy policy of the world.
Many European countries have decided to close their nuclear plants
and freeze their nuclear programs due to risk considerations.
However, research and development of fast-breeder nuclear
reactors and nuclear fusion energy continue in several countries inorder to acquire a stable, long-term energy supply, although they
have not come into commercial use.
1

J. R. McNeill, Something New Under the Sun: An Environmental History of theTwentieth Century (Norton, New York, 2000).
2
http://www.energybulletin.net/3064.html
3
http://www.arabicnews.com/ansub/Daily/Day/041117/2004111709.html
4
United Nations, World Population Prospects, The 1998 Revision; and estimates by52

the Population Reference Bureau.
Campbell and Laherrere, Scientific American, 78-84, March 1998
6 http://www.albany.edu/geosciences/oilngas.html
7
A report on Energy Trends, ExxonMobil Feb. 2004
8
H. Matsumoto, Research on solar power station and microwave powertransmission in Japan: Review and perspectives, IEEE Microwave Magazine, vol.
3, no. 4, 36-45, December, 2002.
9
J. Houghton, http://www.st-edmunds.cam.ac.uk/cis/houghton/lecture4.html
10
S. Arrhenius, Phila. Mag. 41, 237 (1896).
11
J. T. Houghton et al., Eds., Climate Change 2001: Scientific Basis (Cambridge
Univ. Press, New York, 2001). The PDF version is available on the web;
http://www.grida.no/climate/ipcc_tar/
12
M. I. Hoffert, C. Covey, Nature 360, 573 (1992).
13
B. Metz et al., Eds., Cilmate Change 2001: Mitigation, The PDF version isavailable on the web; http://www.grida.no/climate/ipcc_tar/
14
http://www.research.noaa.gov/climate/images/carboncycle_co2mm.jpg
15
N. Nakicenovic and R. Swart, Eds., Special Report on Emission Scenarios, IPCC,
2000. Summary for policy makers is available on the web;
http://www.grida.no/climate/ipcc/spmpdf/sres-e.pdf
16
Global Energy Perspectives, IIASA/WEC, 1998.
17
http://unfccc.int/essential_background/kyoto_protocol/items/3145.php
18
Economies in Transition: Countries of the former Soviet bloc - the Soviet Union
itself and the formerly communist states of central and eastern Europe.
19
Renewables in Global Energy Supply, International Energy Agency,
http://www.iea.org/
20
U.S. Department of Energy, Energy Efficiency and Renewable Energy,
http://www.eere.energy.gov
21
Press releases, 5 March 2004 and 7 March 2005, World Wind Energy Association,
http://www.wwindea.org/
22
Thorpe, T W. "An Overview of Wave Energy Technologies", A report produced
for the Office of Science and Technology, AEA Technology Report Number
AEAT-3615, 1998.
23
The ATLAS Project,
http://europa.eu.int/comm/energy_transport/atlas/homeu.html
24 National Research Council, 2001, Laying the Foundation for Space Solar Power:
An Assessment of NASA's Space Solar Power Investment Strategy, National
Research Council, Washington, D.C.
5

53

Chapter 2 Solar Power Satellite
2.1 SPS Features
2.1.1
Basic Concept
The concept of the Solar Power Satellite (SPS) is very simple. It
is a gigantic satellite designed as an electric power plant orbiting in
the Geostationary Earth Orbit (GEO, see Fig. 2.1.1). It consists ofmainly three segments: a solar energy collector to convert the solar
energy into DC (direct current) electricity, a DC-to-microwave
converter, and a large antenna array to beam the microwave power tothe ground. The solar collector can be either photovoltaic cells or asolar thermal power generation. The DC-to-microwave converter of
the SPS can be either a microwave tube system and/or asemiconductor system, or their combination. The third segment is agigantic antenna array. The power beam must be controlled
accurately to less than 0.0005 degrees.
The SPS system is composed of a space segment and a groundpower receiving site. The latter uses a device to receive and rectify
the microwave power beam. The device is called a rectenna(rectifying antenna). The rectenna system converts the microwave
power back to DC power and is connected to existing electric power
networks. The electricity sometimes can be converted to other formsof energy such as hydrogen.

Fig. 2.1.1 Solar Power Satellite (Artist’s Concept)©RISH, Kyoto University
55

The SPS system has that advantage of producing electricity withmuch higher efficiency than a photovoltaic system on the ground.
Since SPS is placed in space in GEO, there is no atmosphericabsorption, the solar input power is about 30% higher density than
the ground solar power density, and power is available 24 hours aday (except for 70 minutes maximum during 42 days near theequinoxes as shown in Fig. 2.1.2) without being affected by weather
conditions. It is confirmed that the eclipses would not cause aproblem on a grid because their occurrences are precisely predictable.
Solar flux is approximately eight times higher in space than thelong-term surface average on the ground if the insolation is 4
kWh/m2/day.1 With 50% MPT efficiency, this gives a net average offour. For the terrestrial system, however, the efficiency, additional
space due to the loss explained in the next subsection, and cost of thestorage system should be considered in order to supply electricity 24
hours a day. This ratio would become higher depending on theefficiency since 100% efficiency is assumed in the storage system.
The possible SPS system issues to be discussed are the microwave
power beam impact on the existing communication networks andbio-bodies.

Fig. 2.1.2 Daily duration of eclipses as a function of the date.256

2.1.2

Clean CO2-free Energy and energy source

Table 2.1.1 Comparison of relative CO2 emissions fromdifferent electricity generation systems (units: g CO2 /kWh)Generating system
Operations Construction Total
SPS (baseline scenario)
020
20
SPS (breeder scenario)
0
11
11
Coal
1222
3 1225
Oil
844
2
846
Liquefied Natural Gas (LNG)
629
2
631
Nuclear power
19
3
22
The CO2 emissions per kWh are compared for SPS, various fossil
fuels, and nuclear power in Table 2.1.1. 3 The CO2 from theoperations of the fossil fuel power generation systems mainly comesfrom burning of fuel, whereas the CO2 emission from nuclear power
plants mainly comes from the use of energy to produce nuclear fuel.
Almost zero CO2 emission is expected from SPS operation. As aresult, the SPS System would release less CO2 / kWh than nuclear
power generation. The breeder scenario in SPS means installed SPS
units supply electricity to produce additional SPS units.3
A huge, clean power source is to be developed for sustainableeconomic activities with a sufficient suppression of CO2 emission.
Only solar technologies can provide such a huge, clean power sourcein the near future. The terrestrial photovoltaics, wind, geothermal,
and other natural resources depend on the environmental conditions
and are neither stable nor sufficient.
2.1.3 Comparison with Terrestrial Photovoltaics
One may compare the output power from a space-based solar
power system with that from a terrestrial photovoltaic array with thesame area as the SPS rectenna area.4 For such a comparison, an
average insolation5 of 5.7kWh/m2/day (in Phoenix, Arizona, or Las
Vegas, Nevada, USA) and a conversion efficiency of 10% areassumed. In addition, the output power from the SPS rectenna isalmost same as that of DC from a terrestrial array system with thesame area lighted by sunlight.
The terrestrial photovoltaic system provides reduced construction
costs. In addition, terrestrial systems can be installed on roofs andare compatible with other purposes such as factories, shopping malls,
57

and parking lots. Since the terrestrial system directly converts thelight energy into the DC power, there is no concern about the
influence of microwave exposure. There is no impact on the use of
the radio spectrum, or on the night sky. Furthermore, the SPS needs
more research for the microwave exposure effects on plants, birds,
and animals to obtain public acceptance. The output from theterrestrial system is, however, affected by the daily and seasonalvariation of insolation. This variation correlates to some degrees withthe local demand for electric power. Energy storage systems must be
added for base-load power applications.

Fig. 2.1.3 Minimum insolation by two-axis tracking flat plate6
No matter how efficient or inexpensive photovoltaic cells become,they cannot overcome the major difficulty of relying on solar powerelectrical generation from panels based on the ground.7,8 Terrestrial
photovoltaics can work only when the sun shines, and the power isgenerated at full capacity during daylight hours when there is no
cloud cover. Therefore, they are intermittent and unreliable unlessequipped with storage systems. If one wishes that power from thesun could be collected 24 hours a day, 365 days a year andtransmitted with ease to any point on the globe, one would needeither further development of terrestrial energy storage technology or
another source of electricity. The SPS system offers such a
possibility except for a short time around the equinox as shown in58

Fig. 2.1.2. Another advantage the SPS system has over theland-based systems is absence of the atmosphere that absorbs theSun’s light. This increases the amount of light reaching eachphotovoltaic cell by approximately 30%. This nearly-continuous
availability and high output mean that SPS can be relied upon as abase-load power source.
Required Solar Cell Area

Ratio

Full sunNight

Storage loss

35
30
25
20
15
10
5
0
1

2

3

4

Insolation (kWh/m2/day)Fig. 2.1.4 Required solar cell area when a terrestrial
solar power system is used as base load.
Since 1 kW/m2 can be obtained at full sun, we assume 24
kWh/m2/day as base load power, which corresponds to 24 hour fullsun per day. If the terrestrial system is used as a base load, the impact
of introducing the storage system should be taken into account. Thevery large solar call area becomes necessary as shown below.
1.

2.

In order for the storage system to supply enough energy evenafter rainy days, the minimum insolation should be taken into
account.
As an example, the minimum daily insolation in the US isshown Fig. 2.1.3, where flat plate tracking the sun in both
azimuth and elevation is assumed. This becomes much worse ifhorizontal plate is assumed. Let’s assume the insolation, say 2
kWh/m2/day. Then in order to obtain 24 kWh/m2/day, thenecessary power for the rest of a day, 22 (=24-2) kWh/m2/day,
59

3.
4.

must be stored.
If the storage efficiency is assumed to be 80%, the latterbecomes 28 (=22/0.8) kWh/m2/day.
In order to store this power under the insolation of 2
kWh/m2/day, the necessary solar cell area is 15 (=( 28+2)/2)
times of that of the full sun operation.

In order to use the terrestrial photovoltaic system as base load, as
many as 15 times of the area for the full sun operation is necessary if
the insolation is 2 kWh/m2/day. This is too costly. This ratio is shownin Fig. 2.1.4 as a function of insolation.
The maintenance of the rectenna on the ground is easy andrequires less cost. The GEO satellite does the solar tracking in space.
Since it remains in a stationary position in the sky relative to the
Earth, no tracking by the receiver is necessary. Therefore, there are
no moving parts that raise the cost of maintenance. The land
occupied by the rectenna can be used for another purpose. About
80% of the sunlight reaching the rectenna goes through the wire
array to the ground surface.
The rectenna is extremely efficient in energy conversion; about
80% of the energy received at the ground is converted to usable
electricity. The maximum energy density at the center of the radio
beam is one tenth of the maximum sunlight energy rate, as measured
at high noon in the desert. The density is less than the safe leveloutside the rectenna site. Thus the total SPS energy arriving at therectenna site would be a fraction of the solar energy that arrives at
each square meter of the site.
2.1.4 Economics
There are four technological challenges in the standard scenario
for the SPS9: the PV module costs, the efficiency of microwave
power transmission (MPT), the mass per peak kilowatt of the solar
modules and the transmission system, the launch costs, and themaintenance costs to replace aging components. In particular, theMPT efficiency and the space module weight per kilowatt are purely
technical issue. The targets are an efficiency about 50% for total
MPT (DC-microwave-DC conversion), $150/kg for launch cost, and1kg/kW for space module. The SPS cost estimation is based on theseassumed targets. If these targets are met, the power generation cost of
the SPS is estimated to be 0.1 to 0.2 dollar per kWh. Innovative radio60

wave technologies have to be developed because improving MPT is
most critical to reducing SPS cost. Whether the cost is the mostimportant reason to abandon the SPS development should also be
discussed. It may be necessary to choose to develop a clean newenergy source by paying some cost for the sustainability of oursociety.
Continuous developments would be necessary for innovativetechnologies, especially radio wave technologies.

2.2 SPS Systems
In the over-all SPS System, the output of the photovoltaic cellpanel is converted to microwave energy, transmitted to the groundrectenna system, and converted back to DC. The aperture of a
microwave transmitting antenna array can be designed with freedomof parameters such as the microwave operating frequency and theantenna element spacing. The dimensions of the rectenna site on
the ground depend on the transmitting antenna size and the beam
(power) collection efficiency.
Assuming 70% conversion rate in
the space segment, 90% beam (power) collection efficiency, and 80%
conversion rate in the ground segment, the estimated over-all
efficiency from DC (output of the solar panel) to DC (output fromthe rectenna system) is approximately 50%.
Fetter 10 concluded "The probability the SPS could produce
electricity more cheaply than solar arrays on earth is so small thatany expenditure of funds and development on this concept would be
unwise and unwarranted." A clear objection to this paper was,however, published by Smith11 in the same journal.
2.2.1
Space Segment
The SPS space segment consists of solar cells, RF circuits andantennas, a sensor for the pilot signal, and a control unit for beam
forming and retrodirectivity, and circuit power supply. A 1 GW
SPS power plant has the following typical dimensions. The area of asolar cell panel is approximately 10 km2 (2km x 5km) for productionof 2GW DC power with the solar cell conversion efficiency of 15%.
The transmitting antenna array will typically be 1km in diameter.
The aperture distribution of the transmitting antenna is determinedsuch as uniform profile or Gaussian profile based on the required
beam collecting efficiency. Assuming an antenna element spacingof 0.75λ=3.8cm at 5.8GHz, a radiator weight density of 2.69g/cc, and61

2

160 antenna elements, one could get 9.6 kg/ m with this design
approach.
2.2.2

Ground Segment

Fig. 2.2.1 Typical power density at a rectenna site
(1kmφ TX antenna with 10dB Gaussian power distribution)
A typical rectenna site is 4 km in diameter for a transmitting
antenna diameter of 1km operating at 5.8 GHz. Under these
conditions, 93% of the transmitted power is collected. The peakmicrowave power density at the rectenna site is 27 mW/cm2 if a
Gaussian power profile is assumed for the transmitter. The beam
intensity pattern has a non-uniform distribution with a higher
intensity in the center of the rectenna and a lower intensity at its
periphery as shown in Fig. 2.2.1. The safety requirement for the
microwave power density for humans is set to 1mW/cm2 in mostcountries, which is satisfied at the periphery.

2.3 SPS Key Technologies
2.3.1
Launch and Transportation
The SPS is a gigantic space power station of ten thousand tons
orbiting in Geostationary Earth Orbit (GEO). This is one hundred
times larger than the present international space station in Low-Earth
Orbit (LEO). Therefore, economical launch and transportation
62

vehicles for massive material, such as the commercially available
Falcon 9 from SpaceX, or other private commercial transportation
providers, are required in order to realize an SPS that could provide
power from space at a reasonable cost. Ariane 5 (Europe) can lift 18
tons; H-IIA (Japan), 10 tons; and Atlas IIAS (USA), 8.64 tons to
LEO. The launch cost (FY’94) of the Ariane 5 is 118 to 130
M$ (million dollars), and that of Atlas IIAS is 110 to 142 M$. A
Chinese rocket “Long March” is more economical. However, the
SPS will be constructed for a long period and its cost cannot be
estimated with these present rockets.
For the launch and construction of SPS, the following two vehicles
are to be developed. One is a Reusable Transport Vehicle to transportheavy materials, at a reasonably low cost, to a LEO where assembly
will be conducted. The other is an Orbital Transport Vehicle to lift
the SPS from the LEO to the final orbit (GEO). These two rocket
technologies are essential for realization of the SPS system.

Fig. 2.3.1 Future generations of transport systems proposed in USA14
2.3.1.1 Launch from Ground to LEO
63

Two transport systems are considered in the NASA's reference
system:12 (1) Heavy Lift Launch Vehicle (HLLV) and (2) Personnel
Launch Vehicle (PLV). NASA is considering the use of methane(CH4) and oxygen (O2). They assume that the gross lift-off weight of
the HLLV is 11,040 tons with a payload to the LEO of 424 tons.
A Japanese research group assumed two kinds of transport systems
and simulated a launch cycle.13 One is a transport system with a
50-ton payload to LEO, the other is a 500-ton payload to LEO. As
the weight of the first Japanese SPS model was 29,000 tons, 58
launches will be needed in one year to launch all materials for the
SPS construction.
A new rocket to launch heavier materials to LEO is necessary forthe realization of SPS. Future generations of transport systems havebeen studied14 without consideration of the SPS project (Fig.2.3.1).
The SPS requires the 2nd or 3rd generation of RLV (Reusable Launch
Vehicle) for supplying power from space at a reasonably low cost.
2.3.1.2 Transportation from LEO to GEOThe SPS is considered to be assembled in LEO and transported to
GEO by solar electric propulsion orbital transfer vehicles (EOTV).
To this end, a high-power magnetoplasmadynamic (MPD) thruster
was designed, built and tested in the SCTM (SSP Concept andTechnology Maturation) program, which is an SPS research programin the USA in FY2001.15 NASA Glenn Research Center’s groupdeveloped 50 kW-class Hall thruster for construction of SPS. Manyof ion thrusters have been developed for other purposes. The
Japanese satellite “Hayabusa” was launched to deep space on May 9,
2003. Hayabusa used a microwave discharge ion engine systemcalled, “μ10”. The accumulated operational time of the “Hayabusa”
is a world’s record.16
A combination of a heavy load transport system to LEO andtransportation from LEO to GEO by the EOTV should be considered
as a solution to economically constructing the SPS. The combination
will be determined based on evaluations of the degradation of solar
cell efficiency by the radiation belts (Fig. 2.3.2) and damage of SPS
system by space debris (Fig. 2.3.3). Launch from the Earth is more
expensive than from LEO to GEO. The problem of the EOTV is its
low speed. Usually the EOTV carries materials from LEO to GEO
half a year to over one year. The exposed time in the radiation belts
and space debris with the EOTV is much greater than that with an
64

RLV. A number of space debris collisions depends on the exposure
time and the size of the payload. The degradation of the efficiency ofthe solar cells depends on the exposure time and the remaining factorof the solar cells. The relationship between the transportation systemand the degradation of the solar cell efficiency was simulated,yielding the following results.17

Fig. 2.3.2 Deterioration of efficiency of the solar cells during 6 months
Altitude vs. Remaining Factor -Fig. 2.3.3 Space debris flux below 2000
65 km (Orbit inclination 0 degree, AD2030)

(1) SSPS materials, except thin-film cells, are transported to GEO byreusable high-thrust OTVs (HOTVs) and solar electric propulsion
orbital transfer vehicles (EOTVs) and assembled there. Thin-filmcells are transported to GEO in a short time by HOTV in order toavoid cell degradation. The solar paddle of the reusable EOTV is
used up after one round trip to GEO. The remaining factor of thecells on the EOTV must exceed 0.6 after one round trip. The
result of the 1MeV-electron irradiation test of a-Si cells (see [4])indicates significant degradation at a fluence 18 of 5×1015/cm2.
Since the fluence accumulated for 30 years on GEO is about
1.5×1015/cm2, this a-Si cell would be acceptable.
(2) If the remaining factor after 10 years on GEO is improved, RLV
transportation of massive amounts of materials to the lower
starting orbits can be reduced. The amount of the RLVtransportation becomes flat when the remaining factor after 10
years on the GEO is between 0.93 and 0.94. We call this the“Critical Remaining Factor (CRF)” because the effect of the highspecific impulse of the EOTV balances the influence of the cell
degradation. If a remaining factor greater than the critical factor isrealized, it is optimum to start the EOTV from an altitude of
500km.
(3) CIGS (copper indium gallium di-selenide) cells have a practicallyinfinite lifetime in space as far as remaining factor is concerned.19
Therefore, degradation by radiation can be minimized by using
CIGS cells, although both indium and gallium resources are in
short supply.
(4) If the degradation characteristics of the thin-film Si cells cannot
be improved, a propulsion system with a specific impulse
exceeding that of the LOX/LH2 engine is required for the HOTV.
The solar thermal propulsion and the laser propulsion are
candidates. The minimum RLV transport amount for the SOTV
(LOTV) is 2.04mreq at 8000km (1.68mreq at 9000km).20(5) The assembly altitude should exceed 3000km in order to reduce
the frequency of the debris impacts to a safe level, and the SSPS
should not be assembled at altitudes between 3000km and11,000km in order to avoid degradation of the cells. Therefore, theassembly altitude is limited to above 11,000km and assembly at
GEO would be appropriate.
To construct the SPS system, it is necessary to develop an
economical large-capacity transport system.
66

2.3.2
Solar power generation system
To realize a commercial SPS, we have to resolve the following
three technical issues regarding solar cells.
1.
weight reduction
2.
cost reduction
3.
mass production feasibility
2.3.2.1 High Efficiency Solar Cells
Si and GaAs solar cells were studied and adopted in NASA's
reference system.12 They assumed that the efficiency of Si (GaAs)
solar cells is 17.3% (20%) at AM021 28º Celsius.
The efficiency of the thick-film solar cells is theoretically limited to20%, but they are heavy. In contrast, thin film solar cells for space
applications with amorphous silicon (a-Si) or with CuInGaSe (CIGScell) are expected to be much lighter, although their efficiency will
be lower than that of the thick-film solar cells.22 The CIGS cells
have the significant advantage that the remaining factor in the
degradation of the solar cell efficiency is almost 1.23
Instead of a-Si, II-V class elements can be used for the solar cells.
After the US “fresh look study,” the combination of small III-V classelement cells with concentrators was extensively studied in SPS
research groups.24 Their main advantage is high efficiency and lower
cell weight. They need a concentration factor greater than several
hundreds.
The concentrator is important in another aspect. Most of thepresent SPS models adopt a sandwich structure; one side is the solar
panel and the other side is a microwave transmitter. In these models,the area of the solar cells is so limited that a light-concentration
technique is needed. The concentrator will play a key role inrealizing bi-directivity: the solar cells on the front plane pointing to
the Sun, the microwave transmitter on the back plane pointing to the
Earth.
2.3.2.2 Mass Production Feasibility
The SPS requires more than 1GW solar cells for a single produced.
The world production of cells for use on the ground was 391MW in2001 and 1,194MW in 2004 (Fig. 2.3.4). It’s a very high growth rateof mass production of the solar cells. In today's world, more than 80
percent of capacity is furnished with single crystal and polycrystal
67

silicon solar cells (Table 2.3.1). CIGS and a-Si thin film solar cells
will be the major sources around 2010. Information regarding mass
production, deployment accumulation, and cost of solar cells ispresented in Figs. 2.3.5 and 2.3.6.

Fig. 2.3.4 Solar Cell/Module Production Volume (Worldwide)
Source: PV News (March 2005)
Table 2.3.1 Solar Cell/Module Production by cell technology (2003).

68

Fig. 2.3.5 Prediction of Solar Cell Production (From NEDO Website)

Fig. 2.3.6 Increasing applications and decreasing cost of solar cells(From NEDO Website)
2.3.2.3 Solar thermal power generation
Solar thermal power generation has a great potential since higher
efficiency and compactness is expected in the future compared withthe photovoltaic generation. It will, however, be necessary to solve
such problems as precise sun pointing control, light concentration,
heat rejection, and life extension. The photovoltaic generation
method will be put into practical use in initial SPS, and the solar
69thermal power generation method will be used after it will become
matured.
Especially, the solar Brayton thermal power generation system ismost developed and possible among the thermo-mechanical systems.
Since NASA executed the research and development as a power
supply for the International Space Station, it has technicalbuildup.25,26 Brayton cycle systems utilize a turbine, compressor androtary alternator to generate electrical power using an inert gas
working fluid. A recuperative heat exchanger between the turbine
discharge and receiver inlet is used to improve cycle efficiency. Their
unit conversion efficiency of 28% and the system conversion
efficiency of 17% are achieved under the present situation.27,28
On the other hand, highly effective systems that combines static
thermoelectric conversion devices that are characterized by their longlife and radiation resistance are studied.29,30,31 The compound deviceof the thermal electronic power and AMTEC (Alkari Metal
Thermoelectric Energy Conversion) is a representative and ispromising as a power supply for the Orbital Transfer Vehicle for alarge amount of cargo shipment such as SPS.
2.3.3

Thermal Control Technology

Fig. 2.3.7 Sun light concentration rate and balance temperature of generator(α absorption ratio of solar power; ηG, efficiency of solar cell)32

70

Recently, SPS designers have noted the importance of a thermal
control technology because recent SPS models adopt solar cells withconcentrators in order to reduce the weight of solar cells and also
adopt the solar cell – microwave ‘sandwich’ system modules in orderto reduce heavy power lines. The concentrators and the sandwich
modules cause thermal problems because of higher solar power
inputs to the limited cell area. The reference system designed byNASA and DOE had large heat radiation panels behind solar cells to
avoid the thermal problems. The thermal control technologies areimportant topics in SPS system design. Figure 2.3.7 shows therelationship between the concentration rate and the surface
temperature of solar cells with normal heat radiation planes.32 A
concentration rate of n=2 means that the solar power input is twotimes greater than the normal solar power input. Generally speaking,solar cells have to be used below 373 K. The SPS system requires
more reduction in weight and size of the solar cells to reduce thetransportation cost. However, the results shown in Fig. 2.3.7 indicatethat the concentration rate is limited by the temperature of the solar
cells, which means that there is a limit in downsizing of the solar
cells. It also means that we have to develop a new heat radiation
system to concentrate more solar power on the solar cells. Figure
2.3.8 Effects of a spectral filter set between the sun and the solar
cell33Figure 2.3.8 Effects of a spectral filter set between the sun and thesolar cell34
71

As a countermeasure, a surface cover with a wavelength selection
function is proposed in Japan to reduce heat input. The concept of the
wavelength selection is shown in Fig. 2.3.8. Essentially, unwanted
radiation is reflected so that it does not reach the solar cells. Thus,
the solar cells can operate more coolly and more efficiently.
In Fig 2.3.9, three types of selectors are examined. Types 1, 2,
and 3 mean a-Si:H, CdTe, and CIS types. In this study, we consider
methods that have a quantum efficiency better than 0.5 for the solar
generator and the generation efficiency is assumed to be 15% for
these three types.

Fig. 2.3.9 Spectral sensitivities of solar cells35
The effect of the sunlight concentration rate for each type isstudied in Fig. 2.3.10. Each type releases some unwanted heat fromthe solar panels by thermal radiation. Type 1 appears to be especiallyeffective. Compared with the case without spectral selection, Type 1
eliminates 32 percent of the heat. Type 3 blocks 60 percent of theheat, although its effectiveness is low. When there is no concentrator
on the solar cells, excess heat is about 1 kW/m2, but with Type 1, it isabout 0.32 kW/m2. With Type 3, it is about 0.60kW/m2. It was foundthat Type 1 can dissipate the heat for six-time concentration when
wavelength selection is employed. Without it, Type 1 can dissipate
the heat for two-time concentration.

72

Figure 2.3.10 Relation between sunlight concentration ratio and heatrejection rate (with spectral filter attached)33
2.3.4
Microwave Power Transmission on SPS
2.3.4.1 System parameters of MPT on SPS
A huge phased array antenna with high efficiency must be used in
the SPS MPT system. The phased array antenna is necessary forsteering the power beam to a small rectenna target on the groundwithin 0.0005 degrees even though the transmitting antenna of the
SPS will always move and fluctuate. The power beam must be
generated and transmitted without much loss to maintain economy.
An economic analysis in Japan36 gives the optimum size of thetransmitting phased array of a few kilometers and the optimum
microwave power of a few GW at 2.45 GHz. For the same reason, a
DC-RF conversion efficiency, which includes all losses e.g. in phaseshifters, power circuits, and isolators, is assumed to be more than80%. The beam collection efficiency, which is defined as the ratioof received microwave power at a rectenna site to emitted microwave
power from the transmitting antenna, is assumed to be about 90%.
Absorption by the atmosphere is to be less than 2%.12 The weight isalso an important parameter of the transmitting antenna for the costestimation. The MPT system, which includes the generator, amplifier,phase shifter, and antenna, must weigh less than several kg/kW to73reduce the transportation cost.
Table 2.3.2 shows some typical parameters of the SPS transmitting
antenna. An amplitude taper on the transmitting antenna is adopted inorder to increase the beam collection efficiency and to decrease
sidelobe level in almost all SPS designs. A typical amplitude taper iscalled 10 dB Gaussian. With this taper, power density in the center of
the transmitting antenna is ten times greater than that at the edge ofthe transmitting antenna.
Table 2.3.2 Typical parameters of SPS transmitting antenna37
NASAOld JAXA
JAXA1
JAXA2
DOE
Model
model
model
model
model
Frequency
5.8 GHz
5.8 GHz
5.8 GHz2.45 GHz
Diameter oftransmitting
2.6 kmφ1 kmφ1.93 kmφ1 kmφantenna
10 dB
10 dB
10 dB
10 dB
Amplitude taper
Gaussian
Gaussian
Gaussian
Gaussian
Output power
1.3 GW
1.3 GW
1.3 GW
6.72 GW
420
63
2.2
114
Maximum power
222
mW/cm2
density at center
mW/cm
mW/ cm
W/ cm
6.3
0.22
42
Minimum power
11.4
2
2
2
2
density at edge
mW/cm
mW/ cmW/ cm
mW/ cmAntenna spacing
0.75 λ0.75 λ0.75 λ0.75 λMaximum power
185 W
1.7 W
6.1W
0.95 Wper(97(1,950(540(3.54
one antenna
million)million)million)billion)(Number of
elements)
Rectenna
2.0 kmφ
3.4 kmφ2.45 kmφ10 kmφDiameter
Maximum Power
180
26
100
23
DensitymW/cm2
mW/cm2
mW/cm2
mW/cm2
Collection
96.5 %
86 %
87 %
89 %
Efficiency

74

2.3.4.2 Microwave generators and amplifiers
The technology employed for generating microwave radiation isextremely important for the SPS system. It should be highly efficient,very low noise, and have an acceptable weight/power ratio. A
microwave energy transmitter often uses 2.45 GHz or 5.8 GHz in the
ISM band (ISM=Industry, Science, and Medical). There are two
types of microwave generators and amplifiers, the microwave tube
and the semiconductor amplifier. These have contrasting electriccharacteristics. The microwave tube, such as a cooker-typemagnetron, can generate and amplify high power microwaves (overkilowatts) with a high voltage (over kilovolts). It is very economical.
The semiconductor amplifier generate low power microwave (below
100W) with a low voltage (below fifteen volts). It currently is still
expensive. There are some discussion concerning conversion andamplifier efficiency, however, the microwave tube has higher
efficiency (over 70%) and the semiconductor has lower efficiency(below 50%). The weight of the MPT system is also important for
reducing the transportation cost of the SPS. Microwave tube islighter than a semiconductor amplifier when we compare the weightby power-weight ratio (kg/kW) because the microwave tube cangenerate and amplify higher power microwaves than can thesemiconductor amplifier. Detailed research results concerning thesemicrowave generators and amplifiers are described below.
(1) Phase and Amplitude Controlled Magnetron
The magnetron is a microwave tube suitable for the SPS MPT. The
magnetron is widely used in microwave ovens and is a relatively
inexpensive oscillator (below $5). There is a net global capacity of
45.5GW for all magnetrons used in microwave ovens. Only
magnetrons reach the manufacturing capacity for the SPS system.
However, the cooker-type magnetron cannot be applied for the SPS
because it is only a generator and we cannot control or stabilize thephase and the amplitude. As a result, we cannot construct a phasedarray antenna with cooker-type magnetrons.
Some scientists have noticed that magnetrons are cheap, have highefficiency (over 70%), low noise, and have a high power-weight ratio.
The cooker-type magnetron was considered a noisy device.
However, it have been confirmed that spurious emissions from thecooker-type magnetron with a stable DC power supply are low
75enough and this magnetron can be applied in the MPT system.38
Peak levels of higher harmonics are below -60 dBc, and otherspurious emissions are below -100 dBc. The cooker-type magnetronis used as a voltage controlled oscillator in a phase-locked loop.39,40
The difference between the methods proposed in these papers is how
the phase of the magnetron is controlled. An advanced
phase-and-amplitude- controlled magnetron has been developed at
Kyoto University, Japan. 41 They succeeded in controlling beam
directions with phased arrays with phase controlled magnetrons
operated at 2.45 GHz (Fig. 2.3.11) and 5.8 GHz.42 They have also
developed a light-weight phase-controlled magnetron called COMET,
for Compact Microwave Energy Transmitter, with a power-weightratio below 25g/W.43 The COMET includes a DC/DC converter, acontrol circuit of the phase-controlled 5.8 GHz magnetron, a heatradiation circuit, a wave guide, and an antenna. The power-weightratio of the COMET is among the highest of all microwave
generators and amplifiers.

Fig.2.3.11 Phased Array with 12 Phase Controlled Magnetrons at
2.45 GHz © RISH, Kyoto University76

(2) Traveling Wave Tube Amplifier (TWTA）
This is a high-gain microwave amplifier widely used in television
broadcasting satellites and communication satellites. The TWTA has
a proven track record in space. In 1980, it was not a serious
candidate for SPS use because its efficiency was very low, around30%. However, in recent years, TWTs use techniques called velocity
tapering energy recovery44 to achieve net conversion rates of around
70% (Fig. 2.3.12).45 The TWTA has the following track record inspace: 220W at 2.45GHz at 2.65 kg (the TWTA weighs 1.5kg, thepower supply weighs 1.15kg), and 130W at 5.8 GHz at 2.15 kg (theTWTA weighs 0.8kg, the power supply weighs 1.35kg). Hence, they
can deliver 12g/W and 16.5g/W, respectively.46 They do not include
the heat radiation circuit, waveguide, or antenna.

Fig.2.3.12 Trend of Efficiency of TWTA45
Development trends of the TWT include an MPM and aphased-array TWT. The MPM (Microwave Power Module) combines
the best aspects of TWTs, semiconductor amplifiers, andstate-of-the-art power supply technology into one package. This
makes the MPM a good candidate for space application because it
has high conversion efficiency, and is small size and light.
(3) KlystronThe klystron is capable of delivering very high power (tens ofkilowatts to a few megawatts). However it requires a ponderous
77power supply (it requires a heavy magnet). The klystron was selected
for the NASA-DOE SPS model, because of its high conversion
efficiency (76% if the device alone was considered), low harmonic
emissions, and modest weight. The klystron is often used for uplinks(earth stations beaming to orbital satellites). A commerciallyavailable klystron can deliver 80kW of power at 2.45GHz with apower-weight ratio of approximately 100g/W. In C band,commercially available klystrons can deliver 3.2kW but require a
34kg device (permanent magnet) and a 135 kg power supply. It canachieve 40g/W. However, applications of the klystron for SPS havenot been discussed in the recent SPS research in contrast to
magnetrons and semiconductor amplifiers.
(4) Semiconductor Amplifier
In 1980, semiconductor amplifiers were not serious candidates for
SPS use. However, it has been growing as a promising MPT device
in recent SPS research. There are many applications ofsemiconductor amplifiers to communication systems all over theworld, and there are many researchers and users. The technologies
are making steady progress on supports of researchers ofsemiconductor device, circuit, and systems.
Almost all semiconductor amplifier technologies are forcommunication use. Therefore, we have to analyze theircharacteristics from the MPT viewpoint. Examples of characteristics
of various transmitters for space use are shown in Table 2.3.3.47,48
The spectrum between 2 and 4 GHz is called "S Band" in general. In
all cases, it may seem that semiconductor transmitters are light inweight, but closer study reveals that they were quite heavy with therespect to the actual amount of microwave power they can deliver tothe antenna.
Table 2.3.3 Characteristics of Semiconductor Radio Transmitters
for Space Applications
Satellite
ETS-6 NSTAR INT-7 JCSAT-3 Ref49
Ref50
Efficiency
31%
36%
29%
40%
45%
40%
Output
14W
40W
30W
34W
60w
111W
1.2kg
1.9kg
Weight
2.5kg
1.7kg
1.9kg
1.9kg
31g/W
& g/W
63g/W 57g/W 56g/W
17g/W
85g/W
Frequency 2.5GHz 2.5GHz 4GHz
4GHz
4GHz 2.5GHz
78

The other problem is efficiency. Some reports noted that it ispossible to realize a PAE (power added efficiency = (Pout-Pin)/PDC)
of 54%, efficiency of about 60%, at 5.8GHz. These are the best data
in a laboratory. Semiconductor amplifiers require manufacturability
and high efficiency, including efficiency of power source circuits,
loss of isolator and circuits. The efficiency of the driver stage mustalso be considered if the gain of the final stage is not enough.
Although the cost of semiconductor devices is now high since thesemiconductor device for use in SPS is just now being developed, theprice may be reduced through mass production. Therefore, in order
to meet the requirements of light weight, compactness, and highefficiency for the transmitter, hybrid use of a magnetron withsemiconductor devices is also attractive. Another requirement for the
MPT application of semiconductor amplifiers is linearity. The
maximum efficiency is usually attained at a saturation level, where
the linearity between the input and the output is not guaranteed.
Non-linearity causes strong harmonics that must be suppressed in the
MPT. Therefore, dissolution of such difficult relationship between
the efficiency and the linearity is necessary for the SPS MPT.
One trend in semiconductor amplifiers is the development of a
new semiconductor device with increased output power andefficiency. Many advanced solid state devices have recently beendeveloped and improved. For instance, wide-bandgap devices such
as GaN have significant power outputs particularly at relative low
microwave frequencies of 2.4 and 5.8 GHz. High efficiency withhigh power characteristic for the circuit is essential to solve heatproblems in the transmission part of SPS. A novel circuit
technology using a new device is strongly desired to satisfy the needsof both high power and high efficiency simultaneously.
The other trend is development of a Microwave Monolithic
Integrated Circuit (MMIC) to reduce size and weight, especially formobile applications. Lighter transmitters can be realized with the
MMIC devices. However, MMIC devices still have heat-releaseproblems, poor efficiency, and low power output. However, thetechnical problems are expected to be solved by efforts of manyengineers.
The optimum microwave generator and amplifier have not been
selected yet. A hybrid system combining high power microwave
tubes at the center of the array and low power semiconductor
amplifiers at the edge of the array and/or MPM (microwave power79

module)-like hybrid amplifiers is a possible solution. It is important
to continue the fundamental research and development of eachdevice because it is still a long way before realization of the SPS.
The MPT must satisfy noise requirements to avoid harmfulinterference with neighboring frequencies. Much more research isneeded in this area.
2.3.4.3 Antennas
Various types of antennas on SPS have been considered. The
antenna type is determined in relation with the microwave generator
and amplifier. NASA-DOE’s SPS adopted a slotted waveguide
antenna with klystrons. An experimental SPS called SPS2000 in
Japan adopted a slot antenna connected to semiconductor
amplifiers.51 It is 37 mm thick and has a density goal of 6.72 kg/m2.
Lightweight low profile models with metal posts and 3mm or 12mm
thick have been proposed recently.52 In 1992, a 2.45GHz Japanese
SPS model with dipole antennas and reflectors was proposed.53 It is
expected to slash the antenna element weight from 20g to 10g. The
system, including the case and heat radiator, consists of 64 elements.
It would be 48cm ×48cm ×1mm ×2.69g/cc=620g in size and weight.
2
Thus, 5.5kg/m could be realized. Performance at 5.8GHz would also
be pretty good. Assuming an antenna element spacing of
0.75λ=3.8cm, the same radiator weight density, and 160 antenna2elements, one could attain 9.6 kg/ m with this design approach. A
novel concept of partial drive has been proposed and studied to
drastically reduce the driven elements in an array of small radiators.54
A phased array with middle size parabolic antennas has been
proposed in Japan to reduce of the number of elements.55 Microstrip
antennas can also be applied for the transmitting antenna. However,
the weight of the dielectric base is a problem. The weight of theantennas is also important for reducing transportation cost. There is alight weight antenna for space use, but not for the SPS. NASDA
achieved a 2.8g/m2 antenna in Ka-band. The features are 12 elements(not powered by electricity), two layers, a batch antenna, glass
ceramics with εrr =5, 5x5cm in size, and 7g in weight.

80

2.3.4.4 Matching between microwave generators and amplifiers
and antennas
High Power
Oscillator
AntennaPhase shifter(Isolator)

Multiple
Power
Dividers

Phase shifter(Isolator)

AntennaPhase shifter(Isolator)

AntennaFig. 2.3.13 Implementation of microwave transmission with a highpower microwave oscillator and phase-shifters for high precision controlof microwave beam direction to large angles without grating lobes37

High Power
Oscillator

Multiple
Power
Dividers

Phaseshifter

Lowpower
amplifier

Isolator

AntennaPhaseshifter

Lowpower
amplifier

Isolator

AntennaPhaseshifter

Lowpower
amplifier

Isolator

AntennaFig. 2.3.14 Implementation of microwave transmission with phaseshifters and low-power amplifiers for high-precision control ofmicrowave beam direction without grating lobes37
As discussed in section 2.3.4.1, the optimum to be economic sizeof the transmitting phased array and microwave power are calculatedaround a few km and over a few gigawatts. This means that the
microwave power from one antenna element is much smaller thanthat from one microwave tube or high power (over a several tens
watts) semiconductor amplifier. It also means that phase shifters haveto be installed after the microwave generation and amplification
stage (Fig.2.3.13) if microwave beams are to be steered over 5
degrees without grating lobes. In that case, development of low-loss
phase shifter is very important for constructing a phased-array81

antenna with high efficiency. However, the phase shifter problem
will be solved if the microwave beam will be steered within 0.1
degrees because such many phase shifters do not need to be installed
without grating lobes with a large sub-array. Another way to solve
the phase-shifter problem is to use low-power amplifiers after the
relatively high loss phase shifters (Fig. 2.3.14).
2.3.5
Target detection and beam control
It is important that all of the transmitted microwave power iscollected in the rectenna site on the ground. As described in section
2.3.4, absorption by the atmosphere is to be less than 2%.
Accuracies of target detection and beam forming are very important
in increasing the beam collection efficiency.
2.3.5.1 Retrodirective target detection
A retrodirective target detection technique is adopted in all SPS
designs. Retrodirective detection can be realized by a number ofdifferent techniques.56 The most basic is a corner reflector (Fig.
2.3.15(a)). 57 The corner reflectors consist of perpendicular metal
sheets, which meet at an apex. Incoming signals are reflected back inthe direction of arrival through multiple reflections off the wall of thereflector. A Van Atta array58 is also a basic technique for developing
a retrodirective system (Fig. 2.3.5.1(b)). This array is made up ofpairs of antennas spaced equidistant from the center of the array, andconnected with equal-length transmission lines. The signal received
by an antenna is re-radiated by its pair, thus the order of re-radiating
elements are inverted with respect to the center of the array,achieving the proper phasing for retrodirectivity.

Fig. 2.3.15 (a) Two-sided corner reflector (b) Van Atta Array (c)phase-conjugating array. (Sung et al.,
http://hcac.hawaii.edu/tcwct03/papers/s16p03.pdf)56
82

A usual retrodirective system consists of transmitting andreceiving antennas and a phase conjugate circuit. A signal transmitted
from the target, e.g. from the rectenna site on the ground to the SPS,
is received and re-radiated through the phase conjugate circuit to thedirection of the target. The signal is called a pilot signal.
There are various types of phase conjugate circuits forcommunication use and for SPS use. The UCLA research group uses
phase-conjugating mixers. 59 Phase conjugation with heterodyne
mixing uses an LO signal at a frequency twice as high as the pilot
signal frequency. In the UCLA’s system, frequencies of the pilot
signal and microwave power beam are the same. Kyoto University inJapan has developed two types of the retrodirective systems.60 One
has two asymmetric pilot signals, ωt+Δω and ωt+2Δω, and the LO(local oscillator) signal of 2ωt (Fig. 2.3.16 (a)). The other is pilot
signals with one third of the transmitting frequency and the LO
signal is generated from the pilot signals (Fig. 2.3.16 (b)). The lattersystem solves the problem caused by fluctuation of the LO and thepilot signal that cause phase errors because the fluctuations of the LO
and the pilot signals are synchronous. Mitsubishi Electric
Corporation in Japan developed PLL-heterodyne type retrodirective
system in which different frequencies for the pilot signal and themicrowave power beam, 3.85 GHz and 5.77 GHz, were used.61
These retrodirective systems adopt analog circuits for phaseconjugation. Although it can control the beam at a very high speed,
the beam can be directed to a single direction of a target. The
retrodirective system needs both target detection and beam forming.
Therefore, if we separate the target detection from the beam forming,
we can direct the microwave power beam in any desired direction.
This concept is called a software retrodirective system. A computer isused to detect targets with the phase and the amplitude data from thepilot signal and for beam forming with calculation of the optimum
phase and amplitude distribution on the array. We can apply an
advanced algorithm such as the MUSIC algorithm for target
detection. We need phase shifters for the beam forming instead of thephase conjugate circuit. Kyoto University in Japan and Texas A&M
University in the USA have independently developed software
retrodirective systems.62,63

83

ωtt-φt+(Δωt-Δφ)
ωtt-φt+2(Δωt-Δφ)

ωtt+φtωtt-φt+(Δωt-Δφ)

BPF

BPF

×2

ωtt-φt+2(Δωt-Δφ)2ωtt-2φt+2(Δωt-Δφ)ωtt + φt1/3(ωtt-φt)×3

1/3ωtt

8-way
PD

×2

1/3ωtt+ 1/3φt

2/3ωttBPωtt-φt

2/3ωtt±1/3(ωtt-φt)= 1/3ωtt + 1/3φt
ωtt + 1/3φt

2ωt

(a)
(b)
Fig. 2.3.16 Phase conjugate circuit developed in Kyoto University© RISH, Kyoto University
2.3.5.2 Beam forming techniques
There is a room for forming a desired beam, e.g. with low sidelobe
levels, if the software retrodirective system is adopted in an SPS.
Some algorithms are used for optimum beam forming; these include
neural networks, genetic algorithms, and multi-objective
optimization learning. The optimum is able to suppress sidelobe
levels, to increase beam collection efficiency, and to generate
multiple power beams. We can select goals of optimization andalgorithm freely with considering the time required for calculation.
A phase standard is very important for steering microwave power
beams in a desired direction, both for beam forming with thesoftware retrodirective systems and for retrodirective systems withphase-conjugate circuit. If the phase or frequency standard like thelocal oscillator (LO) signal is different on the array, we cannot steer
the microwave beam to the desired direction. The best way is to use
only one oscillator for the standard of the phase and frequency for aphased array of exceeding one kilometer in size with more than abillion elements, but this is impossible. A better way is to use some84

oscillators on some group of sub-phased array and synchronize theoscillators with each other. Some trials have been carried out. One iswireless synchronization of separated units. The present accuracy ofwireless synchronization is below 0.6 ppm of the frequency andbelow 3.5 degrees of phase error.64 Another is self-synchronization
with some data sent from the rectenna site.65 In this method, thephase in a part of the arrays is changed and a resultant change of the
microwave beam intensity is measured at the rectenna site. Thechange gives us information on phase corrections. Both methods areunder development, and a highly accurate phase synchronous system
is needed for SPS.
The other important point for accurate beam forming is
suppression of phase, frequency, and amplitude errors of theelements in order to maintain high beam collection efficiency and to
suppress the sidelobe level and reduce interference with
communication systems. The calculated phase error of the elements
is required to be below 5 degrees in an SPS-MPT system.66 The
error includes phase errors in every stage, target detection,
microwave generation and amplifier, phase synchronization, andphase shifter. The error also includes a structural error. We need amore accurate MPT system for the SPS.
2.3.6
Rectennas and ground network
The SPS system will require a large receiving area with a rectenna
array and the power network connected to the existing power grids
on the ground. Although each rectenna element supplies only a few
watts, the total received power is in the gigawatts. The existing
power network is much larger: hundreds GW. It is important to study
the rectenna element, array, and networks step by step to realize the
SPS system.
2.3.6.1 Rectenna element
The word “rectenna” is formed from “rectifying circuit” and“antenna.” The rectenna and its word were invented by W. C. Brown
in 1960’s.67 The rectenna receives microwave energy and converts it
to DC electricity. The rectenna is a passive element with a rectifying
diode, and is operated without any extra power source. The rectenna
has a low-pass filter between the antenna and the rectifying diode tosuppress re-radiation of higher harmonics. It also has an output
smoothing filter. The rectenna can have any type of antennas
85

including dipole, Yagi-Uda antenna, microstrip antenna, or evenparabolic antenna. A specific antenna gives an effective aperture
related to its gain. Input microwave power to the rectifying circuit isdetermined by the effective aperture of the antenna and microwave
power density. The rectenna can have any type of rectifying circuit
such as single shunt full-wave rectifier, full-wave bridge rectifier, orother hybrid rectifiers. The circuit, especially the diode, mainly
determines the RF-DC conversion efficiency. Silicon Schottky
barrier diodes have usually been used for rectennas. New diodedevices like SiC and GaN are expected to increase the efficiency.
The single shunt full-wave rectifier is often used for the rectenna.
It consists of a diode inserted to the circuit in parallel, a λ/4
distributed line, and a capacitor inserted in parallel. In an ideal
situation, 100% of the received microwave power should be
converted into DC power. 68 Its operation can be explained
theoretically in the same way as an F-class microwave amplifier. The
λ/4 distributed line and the capacitor allow only even harmonics toflow to the load. As a result, the wave form on the λ/4 distributed line
has a π cycle, which means the wave form is a full-wave rectified
sine form. The highest RF-DC conversion efficiency amongdeveloped rectennas is approximately 90% at 4W input of 2.45 GHz
microwave. Other rectennas have approximately 70 to 90%
conversion efficiency at 2.45GHz or 5.8GHz.

Fig. 2.3.17 Typical rectenna RF-DC conversion efficiency69
86

The RF-DC conversion efficiency of the rectenna depends on the
microwave power input intensity and the connected load. It has the
optimum microwave power input intensity and the optimum load toachieve maximum efficiency. When the power or load is not
optimum, the efficiency is nearly 0% (Fig. 2.3.17). The conversion
efficiency is determined by the diode characteristics. The diode has
its own junction voltage and breakdown voltage. If the input voltage
to the diode is lower than the junction voltage or higher than the
breakdown voltage, the diode does not rectify the input microwave.
As a result, the RF-DC conversion efficiency drops when the input islower or higher than the optimum.
Currently, there is research is being conducted in the field ofrectenna development. It is important to research and develop newrectennas that are suitable for a weak microwave in the microwatt
range that can be used in experimental power satellites and IC tags.
An experimental satellite on LEO will transmit a weak microwave tothe ground because the microwave power and size of transmitting
antenna on the experimental satellite will be limited by the capacityof the present launch rockets. This rectenna should somehow be
integrated with the antenna, and, if possible, a new diode should be
developed. There should also be novel approaches to designingrectifiers.
2.3.6.2 Rectenna Array
The rectennas will be used as an array. Mutual coupling and phase
distribution are usually problems for the antenna array. However, therectenna array is connected in the DC phase, not in microwave phase.
Therefore, its problems differ from those of the antenna array.
It was reported that total output DC power of the rectenna array isless than the sum of individual output DC powers of the rectenna
elements.70 The power decrease for series connection exceeds thatfor parallel connection due to the RF-DC conversion efficiency of
the rectenna elements shown in Fig. 2.3.5.1. The connection
equalizes current or voltage on the rectenna and moves the rectennafrom the optimum point. Simulation and experiments confirmed thatcurrent equalization in series connection is worse than voltage
equalization in parallel connection. 71 There is an optimum
connection for the rectenna array.
The SPS requires a rectenna array with a diameter of 2km.
87Although much has been conducted on rectenna elements, only a few
rectenna arrays were developed and used for experiments. The
largest rectenna array in the world is that used for a ground to groundexperiment in Goldstone by JPL, USA, in 1975.72 The size was 3.4
m x 7.2 m = 24.5 m2. It converted 2.45 GHz microwave energy to 34
kW DC with 82.5% efficiency. A rectenna array with a size of 3.54 m
x 3.2 m was developed for a ground-to-ground experiment conductedby Kyoto University, Kobe University, and Kansai Electric
Corporation in 1994.73 They used 2,304 rectenna elements at 2.45
GHz and investigated the rectenna connection problem. Kyoto
University has several types of rectenna arrays operating at 2.45 GHz
and 5.8 GHz. 74 These sizes are approximately 1mφ. Anotherrectenna array with a size of 2.7 m x 3.4 m was developed for MPT
in a fuel-free airship experiment conducted by the Communication
Research Laboratory (CRL; currently NICT) in Japan and Kobe
University in 1995.75 There is a large gap between these arrays of afew meters in size and the SPS array of kilometers in diameter.
Research on larger-scale rectenna arrays is required.
2.3.6.3 Ground network
It is widely assumed that a commercially feasible SPS is on theorder of gigawatts. It delivers significant electric power and cancontribute to any national power grid.
The technology forconnection to the grid already exists, although the output of the SPS
is a direct current. The output of thermal or nuclear power plant is
AC because they must first drive turbine-generators.
As noted above, an SPS rectenna has no moving parts. We foresee
no problems (economic, technological, etc.) with connecting the SPS
to a national power grid because the SPS is a "steady-state" system.
The output is predictable. Moreover, a GW class power plant issimilar to a nuclear power plant or large hydropower plant. Most of
the grid connection issues, therefore, are the same. The SPS issimilar to a nuclear power plant in that it provides "base" power to a
power grid --- SPS is not intended to meet fluctuating power needs(daily, seasonal, or otherwise). SPS does have some "down time"(seasonal blackouts due to eclipses), but these situations can becompensated with back-up thermal systems.
It is presumed that the SPS is a power source that is put into
service in a national power grid (electric power generation and power
distribution system). When The SPS becomes "on line," accidents
88can occur at either the SPS side or the grid side. It is felt that a large
power source, such as the SPS, is not really a new situation for
power utility companies. The grid is designed to take up the slack if
the SPS dropouts without warning. For example, hydropower plants
can increase their outputs to compensate for the temporary losses,(for example, by releasing the reserved water). In some cases theoutput of the rectenna may lapse. However, the DC power converter
may be able to handle these lapses in most cases -- within a certain
specified range. If the lapse or power failure is too large, thenoutput may cease. If connected to a large national grid, then the gridshould be able to take up the slack, somehow. If an accident occurs
on the grid side, there is potential for trouble for the rectenna (powersource to the grid). The grid may be hit by electrical storms (thunderstorms), but the power failure duration should be very short, shortenough for the SPS to manage with such hits to the grid. However, amajor accident at another power source (resulting output failure forhours or days), may be difficult for the SPS to cope with. Morecareful studies are needed on this matter.

2.4 SPS Research: State-of-the-Art
1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004

ResearchSPS System Research in JapanSJACSPS Concept Development
and Evaluation Plan
(Japanese Translation)

NEDO (MRI)
Research on SSPS

Review of RRLSpecial Issue on SPS

Keidanren
Special issue on SSPS

STA, NASDA (MRI)
Innovative Space Infrastructure

NASDA(Institute of Future Technology)
Research on Economy of SSPS

SpecialIssuesEnergy Review
Space Power
Generation and SPSJAXA (MRI)
Research and Examination of SPS
JETRO
Present Research on SSPS andPossibility of Cooperation
between Japan and US

1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002

1970
NASA Begins
Evaluating SSP

SJACSPS WG

LE-NET
DesignJMF (Institute of Applied Energy)
Research on Energy Technology inSpace
STA (Institute of Future
Technology)
Basic Research on Solar
Power Generation in SpaceMITI (USEF)
Committee on
Examination of
Practicability ofSPS

JMF (SJAC)
Research on SSPS

Central Research Institute ofElectric Power Industry
Examination on Power
Reception on the sea and
Available Power of SSPS

2003

2004

Fresh
Look StudyNASA
Terminates
SSP Studies

SPS Research in US in EU

Concept
Definition
Study (CDS)SSP Exploratory
Research& Technology(SERT)SSP Concept
and Technology
Maturation
Program(SCTM)

Advanced
Concepts
Team(ACT)Fig. 2.4.1 Conceptual and Feasibility Studies of SPS in Japan, USA, andEurope through Committee Activities© RISH, Kyoto University
89

The SPS is the largest application of MPT. In Japan, the USA
and Europe, much research on SPS has been and continues to be
conducted.
Figure 2.4.1 depicts committee activities on SPS feasibility studies
from 1979 to 2004.
The activity began in 1979 when aNASA-DOE report was issued. Since then, committee activities tosurvey the conceptual design and the feasibility of the SPS havecontinued intermittently up to today.
2.4.1 US Research
2.4.1.1 Initiative
It is widely recognized that the first concept of the SPS wasproposed by P. Glaser in 196876 after a series of experiments on
Microwave Power Transmission (MPT).77 He proposed to put two
satellites into geostationary orbit (GEO) so that at least one of themwould be illuminated by the Sun at all times. He depicted solar
photovoltaic conversion to obtain DC and a klystron traveling-wave
amplifier to DC-RF conversion as an example. For solar cells with adiameter of 6km as shown in the figure, about 6GW is obtained if
their efficiency is assumed to be 15%.
2.4.1.2 NASA-DOE model78
Following Glaser’s initiative, the United States conducted an
extensive feasibility study in 1978-1980. The feasibility study wasa joint effort of NASA and the Department of Energy (DOE). They
proposed an improved model known as the NASA-DOE reference
model in 197979 shown in Fig. 2.4.2.
In this model, the 50 km2 solar array collects approximately 70
GW of Sun’s provided energy of 1.37 kW/m2 (137 mW/cm2) at GEO
and generates 9 GW DC power (13% total efficiency). This systemtransmits microwave energy of 6.6 GW at 2.45 GHz from a 1km
diameter antenna (78% conversion efficiency). The SPS would
have 100 million antenna elements for an array antenna with anelement spacing of 0.75λ. A 10dB Gaussian taper is assumed for the
power distribution in the transmitting antenna in order to obtainbetter power collection efficiency. A 10 km diameter rectenna
ground site at the equator collects 5.8 GW (87% power collection
efficiency) and 5GW is sent to the utility grid. There would be 10
billion rectenna elements with 0.75λ spacing. If the rectenna site is90

located at 35º latitude, it would be an ellipse of 10 km × 13.2 km
with an overall system efficiency of 7%.
Safety for life on Earth should be taken into account from the view
point of electromagnetic compatibility. Power density distribution
at the rectenna site is shown in Fig. 2.4.3. Although the power
density is 23 mW/cm2 at the center, it is just 1 mW/cm2 at the edge.
The latter satisfies the safety standard and even the former is aquarter of the solar radiation (100 mW/cm2). Note that solar
radiation is weaker on the ground than in space because of theatmospheric absorption.

Fig. 2.4.2 NASA-DOE SPS Reference Model
The NASA-DOE reference model entailed deploying a series of
60 SPSs into GEO. Each of these SPS was planned to provide
dedicated, baseload power ranging from 5 to 10 GW of continuous
energy. The platforms were envisioned to be deployed through useof a massive, unique infrastructure. This infrastructure included afully reusable two-stage-to-orbit (TSTO) Earth-to-orbit (ETO)
transportation system as well as a massive construction facility in
low Earth orbit (LEO). For the construction, hundreds of astronauts
working continuously in space for several decades would have been91

required. The financial impact of this deployment scheme wassignificant. In 1996, more than $250 billion dollars was estimatedto be required before the first commercial kilowatt-hour could bedelivered.

Fig. 2.4.3 Power density distribution at rectenna site on the ground.
2.4.1.3 "FRESH LOOK" SSP CONCEPTS
Although the SPS research was suspended in the US in 1980
because of its high cost estimated, it was not abandoned because of
its high potentiality as a new power source for the next generation.
According to the pre-set policy of re-evaluation of the SPS with anappropriate time interval, the Fresh-Look-SSP (Space Solar Power)
concepts have been envisioned from 1997 as an improved SPS
reference system.

92

(1) Sun Tower80,81

Fig. 2.4.4 The "Sun Tower" SSP Concept (MEO constellation)
The “Sun Tower” SSP Concept is one of the new models andexploits several innovative approaches to reduce the SSP
development and life-cycle cost, while at the same time broadening
market flexibility.
The concept will entail relatively small
individual system components with an extensively evolvable
modularity as depicted in Fig. 2.4.4. Manufacturing can be 'mass
production' style from the first satellite system. Therefore, thissystem can be developed at a moderate price, ground-tested with no
new facilities, and can be demonstrated in a flight environment witha sub-scale test. This system will initially be deployed in low Earthorbit and later migrate to GEO. It is necessary to achieve extremely
low launch costs (on the order of $400 per kg), with payloads ofgreater than 10 MT; this is consistent with Highly Reusable SpaceTransportation (HRST) system concepts.
The "Sun Tower" SSP concept is a constellation of medium-scale,gravity gradient-stabilized, and microwave-transmitting space solar
power systems as shown in Fig. 3. Each satellite resembles a large,
Earth-pointing sunflower in which the face of the flower is thetransmitter array, and the 'leaves' on the stalk are solar collectors. The
concept is assumed to transmit at 5.8 GHz from an initial operational93

orbit of 1000 km and operate sun-synchronous at a transmitted
microwave power level of about 200 MW. Total beam-steering
capability is 60 degrees (+30 degrees). A single transmitting
element is, therefore, projected to be a hexagonal surface
approximately 5 cm in diameter. These elements are pre-integrated
into sub-assemblies for final assembly on orbit. For 200 MW
transmitted RF power, the transmitter array is an element andsubassembly tiled plane that is essentially circular, approximately
260 meters in total diameter, and 0.5 to 1.0 meters thick.
Sunlight-to-electrical power conversion must be modular anddeployable in units 50 to 100 meters in diameter and with a net 1
MW electrical output. The primary technology option is agossamer-structure based on the reflector with non-dynamic
conversion at the focus (e.g., advanced photovoltaics). These
sunlight collection systems are presumed to be always sun-facing(with the system in the sun-synchronous orbit) and to be attachedregularly in pairs along the length of a structural/power transmitting
tether to the backplane of the transmitter array. Heat problems
occur both at the surface of the solar cell array and in the transmittercircuit. For both cases, heat rejection for power conversion andconditioning systems is assumed to be modular and integrated withpower conversion systems. In the case of the transmitter, heatrejection is assumed to be both modular and integrated at the
back-plane of the transmitter array. Power transmission lines fromthe single central tether attachment point to the backplane areintegrated with the modular sub-assemblies of the array.
The nominal ground receiver for the Sun Tower concept is a 4km
diameter site with direct electrical feed into the commercial power
utilities interface. The space segment is consistent with a variety ofground segment approaches. However, during the early years ofoperations, multiple ground stations would be required to achievereasonable utilization of capacity.
For primary power, aground-based energy storage system would be required, in particularin the early phases of overall system deployment in which only asingle Sun Tower was operational. A pair consisting of a single
satellite and a ground receiver would be sized to a 100 to 400 MW
scale, with multiple satellites required to maintain constant power atthat level.

94

(2) Integrated Symmetrical Concentrator82

Fig.2.4.5 Integrated Symmetrical Concentrator83
Two clamshell configurations were developed: a 24-mirror version
with a 2-to-1 concentration ratio on the solar arrays and a 36-mirror
version with a 4-to-1 concentration ratio. Each mirror is planar,approximately 500m in diameter, and is mounted to the back-planestructure at a slightly different angle to form a segmented clamshell
primary mirror. Since the Integrated Symmetrical Concentrator(ISC) is not an optical imaging assembly, the light reflected fromeach mirror only needs to fall somewhere on the PV array, with agoal of minimizing solar array hot spots. The mast length is sized
so that the focal length of the mirrors is greater than 10km, which
provides a reasonable spot size for the Sun’s image on the PV arrays.
With this focal length and a local surface-flatness requirement of
about 0.5 degrees on the mirrors, hot spots and excessive lightspillage around the PV arrays are minimized. Mirrors on the outeredges of the clamshell, which could experience larger deflections
than those located interior to the clamshell, will reflect their energy
onto interior regions of the PV array to reduce spillage. An initial
ISC concept placed the solar arrays on the back of the transmitter, tominimize power cabling distances. However, the backs of both thesolar array and the transmitter need to radiate heat, and thermal95radiation estimates of a back-to-back configuration are 90 kW/m2.
Hence the ISC configurations presented here have two separated
solar arrays that are each canted 10 degrees.
2.4.2 Japanese Research
Japanese scientists and engineers started their SPS research in theearly 1980s. They conducted a series of MPT experiments including
the world’s first rocket ionosphere experiment in 1983 84 , 85 andexperiments on the ground. 86 They also conducted a series ofcomputer simulations87 and theoretical work88 following these MPT
experiments. After the conceptual study phase, two Japaneseorganizations recently proposed their own models.
2.4.2.1 JAXA Models
The Japan Aerospace Exploration Agency (JAXA) is studying the
SPS conceptual and technical feasibility at different component
levels. It is possible to beam the solar energy down to the Earthusing either microwave (radio) technology or laser (optical)
technology. The microwave method is making especially rapid
progress, but the optical methods invariably have weather-related
issues. In 2001, JAXA proposed a 5.8GHz 1GW SPS model using
microwave technology. Various configurations different from theNASA-DOE model have been proposed, evaluated, and revised.
(1) 2001 Model
In 2001, the first JAXA SPS model was proposed (Fig. 2.4.6). It
consists of the following three parts.
Primary Mirror...............................4 x 6 km2
Secondary Mirror.............................2 x 4 km2
Conversion Module (Sandwich Concept)........2.6 km (diameter)
These three parts are mechanically connected. The Conversion
Module is always pointed at the Earth, but the mirrors must rotate
and constantly receive solar radiation. This presents immense
mechanical engineering challenges.
SPS Conversion Module invariably has two components:(1) Solar panel component (Power Generator) and(2) Antenna component (Transmitter).
The problem is how to put these two components together. The
Sandwich Concept is one solution. In this concept, solar radiationis received on the front side, and microwave energy is radiated from96

the back side. Some joint modules are required. When thisfront/back configuration is used, it becomes a very difficult to releaseheat.
In any event, the Conversion Module has a severe heat dissipation
problem. Excessive heat degrades the conversion efficiency of theentire module. In this JAXA model, the estimated distance between
the mirror(s) and the Conversion Module is 3 to 4 km. A very large
truss is also required.

Fig. 2.4.6 Year 2001 Reference Model89(2) 2002 Model
Primary reflector
Diameter 2.5km ×3.5km, 1000t×2 units

Truss
6km、200t
Towards Earth
Transmitter
2kmφ, 7000t

Refracting lensDiameter 2km
400t×2units

Fig. 2.4.7 Year 2002 Reference Model90
97

The 2002 Model was conceived to solve the main (heat) problemsof the 2001 Model. The 2002 Model is depicted in Fig. 2.4.7. The
Primary Mirror is 2.5 km x 3.5 km. The truss is 6 km long andweighs 200 tons. The conversion module is 2 km in diameter andweighs 7000 tons. A 400 ton lens is also needed (discussed below).
The lens is located between the Primary Mirror and the conversion
module. Unfortunately all these components are mechanicallyconnected.
It was suggested that solar reception and microwave transmission
should be performed on the same surface (front side). This would
free up the back side for heat release. Radiation activity (solar
energy reception and microwave transmission) would occur on one
side, and unwanted heat would be released from the other side.
This is all illustrated in Fig. 2.4.8. As shown in this figure, solar
cells and microwave antennas are all on the same surface, that is,aligned side-by-side.
Antenna aperture

Solar Cells
Fig. 2.4.8 Transmitting antenna with solar cells.
As indicated above, the 2002 Conversion Module with solar
energy reception and microwave transmission on the same side isfeasible but entails the following issues.
1. It is difficult to match the efficiencies of the solar cell and themicrowave generation process. It is hard to find a satisfactory
compromise due to large impedance difference.
2. Modularization becomes necessary to transport the system tospace. Each module would be as shown in Fig. 2.4.8. Unfortunately,
it becomes necessary the need to transmit electric current between
modules when the entire SPS System is assembled in space. This
interaction between modules eliminates all the advantages of putting
everything on the same surface.
3. A complicated refraction lens becomes necessary to direct
sunlight from the mirror to the conversion module. This lens would98

be immensely difficult to design and construct.
Hence, the disadvantages owing to all activities on the same sideor same surface outweigh the advantages of this conversion module.
We must return to the Sandwich Concept, even though it still has theheat release problem. Some kind of technology breakthrough isneeded.
(3) 2003 Model (Formation Flying SPS)91
The NASA-DOE Reference Model hinted at the need for arotating joint.
This mechanism becomes necessary because thePrimary Mirror and the Conversion Module inherently have different
requirements. The Primary Mirror must constantly rotate in three
dimensions to accommodate the Sun. It must reflect sunlight to theConversion Module. However, the Conversion Module that beams
energy down to an Earth station cannot be rotated. It was assumed
by all SPS developers that the Primary Mirror must be mechanicallyconnected to the Conversion Module.

Fig. 2.4.9 Year 2003 Reference Model
In 2003, "Formation Flying", a major breakthrough in SPS
development, was proposed. In this new proposal, the Primary
Mirror is physically separated from the Conversion Module. The
2003 JAXA model is shown in Fig. 2.4.9. It is based on aformation-flight of a rotating mirror system and an integrated panel
composed of a photo-voltaic cell surface on one side and a phased99array microwave antenna on the other side. The lifting force
provided by solar pressure can be used to fly the Primary Mirrors
independently. Formation flying mirrors are used to eliminate theneed for the rotary joints. The whole system becomes moremechanically stable and reliable.
The SPS main body will be placed on GEO, and the two primary
mirrors will be placed a few kilometers north / south of the main
body. The solar collection mirrors receive solar pressure from theSun. Since the primary mirrors are tilted against the GEO plane,
this solar pressure is divided into the horizontal (parallel to GEO
plane) force and the vertical force. The horizontal force should becanceled using some kind of actuators such as the ion thrusters. The
remaining vertical force acts as the lifting force that moves themirrors away from the GEO plane. The mirror also receives thegravitational force caused by the mirror’s orbital motion. If thegravitational force is cancelled by the lifting force generated by thesolar pressure, then the primary mirrors can stay north / south of the
SPS main body, while the primary mirrors are placed on a slightly
inclined orbit against GEO. A matter for future studies is how tocontrol the shape and attitude of such light yet huge structures.
2.4.2.2 USEF Model92,93(1) Design Concept
An SPS with a simple, technically feasible, and practicalconfiguration has been investigated. The Institute for Unmanned
Space Experiment Free Flyer (USEF) proposed a simpler model. A
study team organized by USEF conducted engineering research for
an SPS demonstration experiment using the Japanese launch vehicle
H2A.
An artist’s concept of the Tethered-SPS is shown in Fig. 2.4.10.
The system consists of a large power generation/transmission panel
suspended by multi-tether wires from a bus system above the panel.
The attitude is automatically stabilized by the gravity gradient force
in the tether configuration without any active attitude control. The
power generation/transmission panel consists of perfectly equivalent
modules, which greatly contributes to the low-cost production,
testing, and quality assurance. For the simplest configuration of thepower transmission system, the small antenna element and themicrowave circuit are integrated as a single entity using Active
Integrated Antenna (AIA) technology. Another innovative feature100

of the module is the cableless interface using a wireless LAN system,
which leads to a reliable deployment, integration, and maintenance.
The tethered SPS is an assembly of equivalent miniature tethered
SPS elements.

Fig. 2.4.10 Artist’s concept of the tethered-SPS
Since this system has no mechanism to track the sun for the power
generation, the total power efficiency is 36% lower than that of theNASA-DOE reference model or other sun-pointing types of SPS.
However, the simple concept resolves almost all the technicaldifficulties in previous SPS models. The absence of a moving
structure in a large scale makes this system highly robust and stable.
Since a light concentrator is not used for power generation, alarge-scale power generation area of km size is required, but the heatgenerated in the panel can be released into the space without any
active thermal control.
From the construction view point, this configuration enables
verifying the function of the SPS phase by phase. In most other
SPS models, the concept of phased construction steps has not been
implemented, but is very important for a large space infrastructure.
The miniature tethered element, a part of the commercial SPS, can beused for demonstration experiments in the near future. This
strategy provides a scenario for the development from ademonstration model to a commercial SPS in the technology road
map.

101

(2) Tethered Solar Power Satellite

Fig. 2.4.11 1GW class Tethered-SPS
The SPS proposed by USEF is illustrated in Fig. 2.4.11. This
tethered SPS is capable of 1.2 GW power transmission and 0.75 GW
average power reception on the ground. It is composed of a power
generation and transmission panel of 2.0 km x 1.9 km suspended bymultiple-wires deployed from a bus system located 10 km above thepower panel. The panel weighs 18,000 tons, and the bus system,
2,000 tons 0.1 m thick. The panel consists of 400 subpanels of 100m x 95 m. Each subpanel has 9,500 power generation andtransmission modules of 1 m x 1 m in area. In each power module,
the electric power generated by the solar cells is converted tomicrowave power and used for other control units. Therefore, there
is no power line interface between the modules. The microwave
transmitting antennas are on the lower plane. Figure 2.4.12 depicts
the concept of the power module. The power module can berealized in the plate configuration with thin film solar cells on boththe upper and lower planes and with a planar transmission module
including the small antenna and phase controlled magnetron and/or
the microwave integrated circuit using Active Integrated Antenna(AIA) technology.
The module contains a power processor, microwave circuits, and102

their controller. Each module transmits 420 W maximum of
microwave power. The power conversion efficiencies for the solar
cells and the DC to RF converter are assumed to be 35% and 85%.
The module weighs 5 kg and the specific weight of the module is 12
g/W. These values are two or three times those of the existing
technologies for the power conversion efficiencies andapproximately 10 times less for the specific weight, but areconsidered to be realizable in 20 to 30 years.

Fig. 2.4.12 Power module

Fig. 2.4.13 Overall construction scenario
There is no wired signal interface between the power modules.
The control signal and frequency standard for each module areprovided from the bus system by the wireless LAN. The tetheredsubpanel is composed of a 100 m x 95 m subpanel suspended by four
wires connected to a bus system. The 100 m x 95 m 0.1 m thickpanel is regarded as a solid panel with the required flatness for phase103

control in the microwave power transmission. The subpanel
consists of 950 structural unit panels of 10 m x 1 m x 0.1 m. The
over-all construction scenario is illustrated in Fig. 2.4.13.
The structural unit panels are folded in a package of 9.5 m x 10 m x
10 m that composes a unit cargo transported from the ground to low
earth orbit (LEO) by reusable launch vehicles (RLV). The cargo istransferred to the orbit-transfer vehicle (OTV) in LEO around 500
km and transported to GEO. To avoid the degradation of the solar
cells by trapped energetic particles in the radiation belt, the cargo iscontained in a radiation shield vessel. If we use a 270 MT OTV
equipped with an electric propulsion of 240 N thrust, the cargo istransferred to GEO in two months. The tethered subpanel isdeployed automatically in GEO. After the function test of thetethered subpanel is completed, it is integrated to the SPS main body.
Docking assistant robots that are manipulated from the groundcontrol center will be required for the integration. This strategy
makes it possible to verify the SPS function during the construction
phase from the low power to the full power.
2.4.2.3 SPS 2000
The ISAS (Institute of Space and Astronautical Science）solar
power satellite working group proposed SPS 2000 94 fordemonstration of electric power supply to customers at the earliest
opportunity. The 10MW model will be launched in a 1000-km
equatorial orbit. The SPS 2000 system includes consideration ofsocial, economic, political, legal, public relations 95 and othernon-engineering aspect from the start and was designed in the hopethat other organizations will build other satellites of various designs
to deliver additional power supplies to the same rectenna. The detailis introduced in Appendix B.2.
2.4.3
European Research
Europeans proposed a Sail Tower SPS96,97 (Fig. 2.4.14). The
Sail Tower design is similar to NASA’s Sun Tower SPS but uses athin-film technology and innovative deployment mechanisms
developed for solar sails. The main characteristics are summarizedin Table 2.4.1.

104

Table 2.4.1 Sail Tower Characteristics

Fig. 2.4.14
Tower.

Sail

Each single sail is 150m×150m and is automatically deployed byextending four diagonal light-weight carbon fiber (CFRP) booms thatare initially rolled up on a central hub. The power generated within
the sail modules is transmitted through the central tether to theantenna where microwaves of 2.45 GHz are generated in
mass-produced inexpensive magnetrons.
Slotted carbon-fiber
waveguides mounted on the antenna main structure are used as active
antenna elements. As the phased arrays, several sets of wave
guides radiate the microwave power to the rectennas on the Earthwhere the power is transformed and fed into the existing power
distribution networks. The power intensity across the antennasurface is designed with a truncated 10 dB Gaussian distribution thatminimizes side lobes and scattering.
This technology is much more developed than laser powertransmission and promises much higher system efficiencies withalmost no weather dependency.
In 2003, the Advanced Concepts Team (ACT) of the European
Space Agency (ESA) initiated a three-phased, multiyear program
related to solar power from space. In Europe, terrestrial solar power
is one of the fastest growing energy sectors with high growth rates
sustained over more than a decade and very promising forecasts.
The first phase of the European Program Plan therefore involved the105terrestrial research community and was dedicated to assessing thegeneral validity of space concepts for Earth power supply bycomparing them with comparable terrestrial solar concepts.98,99 In
parallel, the general validity of SPS concepts for space exploration
and applications were assessed by comparing them with traditional
solutions and nuclear power sources.
The first step was taken in August 2002 with the creation of theEuropean Network on Solar Power from Space. It provides a forum
for all relevant and interested European players in the field of SPS,
including industry, academia and institutions.
After the definition of the main aspects of the SPS Programme Plan
with its three phases, the activities were done in parallel withinstudies by the Advanced Concepts Team and by European industrial
and academic contractors.100,101,102,103(1) Integration of Terrestrial Solar Power ExpertiseTwo parallel industrial studies were undertaken. The two
consortia were led by independent energy consultant companies,
which coordinated the space as well as terrestrial solar power
expertise.
(2) Power Consumption Profile
The scenarios were divided into the provision of base-load power
and the provision of peak-load power. For this purpose, thebase-load power was defined as the constant provision of the lowest
daily demand level. The peak load power was then defined as“non-base-load” power as shown in Fig. 2.4.14, which also gives atypical daily power load profile in Europe over one typical weekday.
(3) Supply Scenarios
Solar power satellites are frequently proposed in themulti-gigawatt range, while terrestrial plants are currently proposedin the several MW range. In order to derive the scaling factors forspace and terrestrial solar power plants, different plant sizes ranging
from 500 MWe to 150 GWe for the peak-load and 500 GWe for thebase-load scenarios have been analysed.
(4) Launch Costs
Launch costs are the single most important parameter in assessing
the economic viability of solar power satellites. Any assumption of106

fixed launch costs would predetermine the outcome of the systemcomparison studies.
As a consequence, the launch costs were treated as open
parameters for these assessments between boundaries given by thecurrent launch cost as upper and the fuel costs as lower limit.
In order to overcome the “chicken-egg” problem of the launchfrequency required by the construction of SPS reducing the launchcosts to values required for the economic construction and operationof SPS, a “learning curve approach” was agreed upon by bothconsortia. Starting from current launch costs, a 20% reduction was
assumed by each doubling of the total launch mass (progress rate of
0.8).
In the first step, space and terrestrial plants were compared by
excluding the transportation costs. This comparison and differenceof the total costs determine the maximum costs allowed fortransportation in the space scenario to remain competitive withterrestrial plants.
In the third step, the progress rate was used to determine thereduction of the launch costs due to the required number of launchesof the SPS components for all scenarios. These values were thencompared to the required value to become competitive for a certain
scenario as determined in step two. The approach did not take into
account potential multiplication factors due to possible opening ofadditional markets created by lower launch costs.
For the base-load power supply scenario, one consortium opted as
most likely a system of multiple 220 MWe solar thermal tower units
distributed within the south European sunbelt region (including
Turkey). The other consortium based the analysis on a solar
thermal trough system installed in an unpopulated area in Egypt.
Both consortia considered PV plants as higher-cost alternatives withcurrent technology but with large cost reduction potential for the
2020/30 timeframe.
The system of choice for the peak load power supply of one
consortium was a highly distributed PV-based scenario, where theamount of unused, potentially available and usable building surfaces
were taken into consideration. The other one opted for the same
design as for the base-load solar power plant.
Given the restriction to European scenarios, only geostationary
space systems were taken into account. While one consortium has
opted for wireless power transmission by laser, the other preferred107the 5.8 GHz microwave wavelength. Both concepts rely on
land-based terrestrial receiver sites (instead of sea-based receivers).
In principle, the first phase was not intended to develop new spacesolar power station designs, but to rely on the most advanced
technical concepts proposed (European Sail Tower concept, theconcepts proposed during the NASA Fresh Look and follow-on
studies as well as Japanese concepts). 104,105,106
Because of limited data on concepts relying on laser powertransmission, some further assumptions have been made. The
general outline of the laser-based space plant is a geostationary space
unit with 111 km2 of thin film PV cells augmented by concentratorsof the same area. The 20% efficient system generates 53 GWe in
orbit, feed into a 50% efficient IR-laser generation system at 1.06
μm transmitted with average losses of about 38% essentially
due to beam shaping and atmospheric attenuation to analmost 70 km2 large PV reception site in North Africa. The
ground PV system would have a 20% efficiency for direct sunlight
but a 52% conversion efficiency for the IR-laser beam. Adding
additional 4% collection losses in space and 4% losses on ground,the space segment would deliver a constant supply of 7.9 GWe to theterrestrial power grid.
The comparison results are presented below.
(1) Base-load Power Supply

Fig. 2.4.15 Definition of base- and peak-load (non-baseload) poweras used for the present assessment.

108

In the case of base-load scenarios, terrestrial solar tower plants
with local hydrogen storage capacities promise electricity generation
costs between 9 €cent/kWh for the smallest (500 MWe) and 7.6€cent/kWh for the largest (500 GWe) plants.
Under those conditions, the SPS would not be competitive withthe smallest scenarios even without any launch costs. For the 5
GWe and larger scenarios, the launch costs between 620 and 770€/kg are required for the SPS to be competitive with terrestrial plants.
In case local pumped hydrostorage facilities are available, therequired launch costs would be significantly lower, dropping toroughly one third of these values.
(2) Non base-load Power Supply
For non-base-load scenarios, solar tower plants with local
hydrogen storage capacities have generation costs between 10 €/kWhfor the smallest scenarios to 53 €cent/kWh for the largest (150 GWe)
plants.
Solar power satellites reach potentially competitiveelectricity generation costs only above relatively large plant sizes ofabout 50 GWe.
For the 50 GWe and higher scenarios, launch costs between 155
and 1615 €/kg would be required for SPS to reach a competitive
level with terrestrial plants. If locally pumped hydrostorage
facilities are available, the required transport costs would be loweredby about a factor two.
(3) Energy payback times - primary validity
Space as well as terrestrial solar power plant concepts have been“accused” of violating the fundamental law of every power plant:
generating more energy than necessary for their proper construction.
It was therefore important to assess the exact cumulated energy
demand (CED) of the systems and compare it with the energy output
over their lifetime. The resulting energy payback time provides ameasure for the validity of the concepts as power plants.
There are several methods to assess the cumulated energy demandof any system. The fast but most imprecise method is an energetical
input/output analysis. This method was already partially applied to
SPS systems in the past, in part based on energy estimates derived
from material costs, assuming a reliable €-Joule relationship. If all
the components are known, a material balance analysis can be made,109

combining the masses of all components with their specific energy
demands obtained from specialized databases.
The European analysis relies on a complete material flow analysis,
the most precise method to determine the CED. For some parts ofthe space system for which the data for the exact material flow
analysis were not available, the method of material balance was used,partially based on CEDs provided by specialized databases.
In all considered cases, the energy payback times for space andterrestrial solar power plants were shorter than or equal to one year.
For the Egypt-based terrestrial system, the energy payback times
seem to be slightly higher than for the distributed system in theEuropean solar belt. In both cases, from a purely energetic point,solar power satellites promise a slightly shorter energy payback time,
ranging from 4 month to 2 years depending on the size and theconcept (all including the launchers).
It should be noted that while using slightly different methods anddifferent space concepts, the assessments by the two independentconsortia for the space segments derive almost the same values (3.9
to 4.8 months) despite their different transmission technologies.
The terrestrial scenario based on solar thermal tower plants (local
hydrogen storage) in south Europe leads to energy payback times of
8.4 months, the solar thermal trough case (with pumped
hydroelectric storage) in North Africa has a calculated payback timeof 8.1 to 8.9 months. The energy payback times for the terrestrial
photovoltaic case in north Africa are expected to fall from about 31
months with advanced current technology to 8.3 months based on
2030 PV technology.
The detailed assessments have shown that both space andterrestrial solar plants have extremely short energy payback times
and are, from a purely energetic point of view, attractive power
generators.
ESA will start the second phase of the European SPS ProgrammePlan in late 2005/ early 2006, expected to provide advanced results
concerning the integration of space and terrestrial solar power plant
concepts and laser power transmission techniques.
2.4.4 Worldwide Activities
As world-wide activities, the SPS research groups have initiatedinternational
collaborationsuchasJapan-US
SPS
workshop, 107 International Conference on SPS and WPT, 108110International Astronautical Congress (IAC) Space
Committee,109 and URSI inter-commission working group.
1

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103

118

Chapter 3

SPS Radio Technologies

This chapter describes the present and future research on
SPS-related technologies. Microwave power transmission (MPT),
including beam control, microwave devices, rectennas, measurements,and spin-off technologies are described. Influences of SPS andscientific topics are treated in Chapter 4.
Antenna and power transmission technologies applied to SPS arebasically extensions of conventional array antenna systems. There exist,
however, some essential and important differences, which are itemized
below.
1) For microwave power transmission (MPT), highly efficient energy
transmission between the transmitter and the receiver antennas isrequired. The product of the transmitter and receiver diameters is a
key parameter.1 A huge array is necessary for high efficiency. The
diameters are on the order of kilometers and the number of theirelements is on the order of billions for the SPS. The efficiency isabout 90%.
2) Radiation in an unwanted direction at an angle from the main beam
is called a sidelobe. Sidelobes, which are also called grating lobes,
are equal in amplitude to the main beam if the element spacing
exceeds one wavelength in a uniform linear array. Suppression ofgrating lobes and sidelobes is necessary for safety and in order toavoid interference with communications. Tapering of the output
power distribution or its equivalent in the transmitter antenna array is
one way to decrease sidelobes and increase the transmission
efficiency. However, this makes the antenna and power transmission
system complex.
3) The microwave beam should be correctly directed to the rectennasite. Pointing accuracy 300 m or less from GEO (36,000 km in
altitude) is required for the a rectenna diameter of a few to several
kilometers. This corresponds to 0.0005º.
4) It is desirable to decrease the number of phase shifters as they cause
extra losses and are expensive. It is also important to decrease thenumber of power dividers in the case of vacuum tube transmitters forthe same reasons.
5) Highly efficient and light weight power transmitters with low
harmonics need to be developed. The low weight to power ratio is
important for decreasing the launch cost. The microwave devices for
the SPS power transmitters are either semiconductor devices or119microwave tubes.
6) Received power densities are not constant over the rectenna site.
Rectennas that are highly efficient under various input power
conditions must be developed. Connection of the rectenna output tothe existing power network is another important issue.
7) Measurement and calibration of the huge antennas are essential.
The aperture of a transmitting antenna array of a typical 1 GW SPS
is 1 to 2 km in diameter. The average microwave power density at the
SPS array will be 1 kW/m2. If we use 2.45 GHz (5.8 GHz) for the MPT,
the number of antenna elements per square meter is of the order of 100(400). Therefore, the power allotted to each element is of the order of
10 or 2.5 W/element. The output powers for typical models under the
10-dB Gaussian taper are shown in Table 2.3.2.

3.1 Microwave Power Transmission Technology
How much energy transmitted from the geostationary earth orbit canbe received on the ground? Basically almost all the energy can becollected on the ground through the microwave power transmission(MPT). The transmission efficiency of the energy is about 90% in
typical SPS designs. MPT characteristics are described below.
3.1.1
Use of EM waves to transmit energy
Following the basic concept derived by Goubau and Schwering,2Brown1 showed that the power transmission efficiency can approach
100% if parameter τ=A/(λD) is larger than 2 and the distributions on
the aperture are Gaussian, where A is the geometric mean of thetransmitter and receiver aperture areas (At and Ar), λ is the wavelength,and D is the separation distance between the two apertures. For
example, the efficiency is 99.63% at τ = 2.4 as shown in Fig. 3.1.1.
This can be attained if both diameters of the apertures, dt and dr, λ, andD are 1.4km, 4km, 5.17cm (5.8GHz), and 36,000 km, respectively. If dt= 1km as shown in the previous chapter instead of dt = 1.4km, then τ =1.7. High efficiency is obtained when the power density distribution
over the apertures is close to Gaussian as shown in Fig. 3.1.2.

120

Fig. 3.1.1 (Left) Transmission efficiency as a function of parameter τfor optimum power density distribution across the transmitting antenna
aperture as shown in Fig. 3.1.211,3

Fig. 3.1.2 (Right) Relative cross-section power density distribution
across the transmitting and receiving antenna apertures for various
values of τ as given in Fig. 3.1.1. R is the radius of the transmitting orcollecting antenna and ρ is the radial distance from the center. The
field at the collector extends beyond its edges.1,3121

3.1.2
Application of Friis Transmission Equation
The conclusion in the previous subsection is contrary to the generalbelief that the power density falls off as the square of the distance. The
Friis Transmission Equation can be written asAAPr= 2t r2 = τ 2
Pt
λ D(3.1.1)
where Pr and Pt are the receiving and transmitting powers, respectively.
In the case of the previous subsection, the receiving power is larger
than the transmitting power since τ > 1. This unrealistic situation
occurs because D < 2dt2/λ, that is, in the near field or Fresnel zone.4 It
should be noted that the received power density is not constant and has
a distribution with a maximum at its center as shown in Fig. 2.2.1 evenin this geostationary distance. The Friis transmission equation is notapplicable. In a case of communications, for example, parameters for a
Japanese broadcasting satellite, dt = 0.73m, dr = 0.37m, λ = 0.025m,
and D = 3600km, then τ = 2×10-7 << 1 as expected.
3.1.3
Characteristics of microwave power transmission
Characteristics of the microwave or wireless power transmission arepresented below.5,6(1) Applications of MPT
The main application is an SPS. Some other applications are
explained in Section 3.5 (Spin-off technologies). More applications are
introduced in Appendix A.
(2) Characteristics of the MPT signal
The bandwidth of the SPS is quite narrow since an essentiallymonochromatic wave is used without modulation. An interference
assessment at 2.45 GHz was published in IEEE Microwave Magazine.7
If power flux densities of harmonics of the WPT frequencies are
regulated by the ITU power flux density (PFD) limits, some modulation
might be necessary.
(3) Category of spectrum: ISM, or other?
The industrial, scientific and medical (ISM) bands at 2.45 and 5.8
GHz, which are common in frequency allocation throughout the world,
have been used for WPT applications for demonstration andexperimental purposes. The 2.45 GHz ISM band (2,400-2,500 MHz)122

and the 5.8 GHz ISM band (5,725-5,875 MHz) are allocated to various
services. Recently, the 2.45 GHz ISM band has been widely used forRadio LAN (IEEE 802.11b and g) applications. The 5.8 GHz ISMband is also heavily used for various applications. The 5,725-5,850
MHz band is allocated to Radiolocation service and is expected to be
used for Dedicated Short-Range Communications (DSRC) applications
described in Recommendation ITU-R M.1543-1. The 5,850-5,925
MHz band is allocated to Fixed/Mobile services and is used for
terrestrial ENG (Electronic News Gathering) in some countries
including Japan. The upper half, 2.45-2.5 GHz cannot be used for SPS
as the second harmonic is allocated to radio astronomy (4.9-5.0 GHz).
It should be noted that these commercial applications are designed at
low cost and are quite vulnerable to interference.
(4) Most suitable frequency bands
Suitable frequencies for Space-to-Ground WPT such as an SPS andrelated satellites are 1-10 GHz (the radio window) in order to avoid
atmospheric absorption and ionospheric scintillation as hightransmission efficiency is essential for SPS. Since the ISM bands arewidely used in various fields, however, frequencies other than thesebands should also be taken into consideration. As compatibility withwireless communications is one of the most important issues,interference simulations will be conducted in the future. Suitablefrequencies are different in applications other than Space-to-Ground
WPT.8(5) Effects on radio propagation
The SPS must radiate at intensities lower than the safe level(1mW/cm2) outside the reception field. No effect on the propagation
has been found except for a case under an intense electric field,9although the intensities exceed the safe level. Near the satellite,
however, the power density will be high and its effects on theionosphere or the magnetosphere shall be examined experimentally.

3.2 Microwave devices
Microwave semiconductors and vacuum tubes are reviewed.
3.2.1

Microwave semiconductors
123From manufacturing points of view, semiconductor technologies
being advanced currently should benefit the SPS technology. Silicon
based devices and III-V or other compounds have been investigatedextensively by the electronic industry and solid state research groups
all over the world. Recent progress in GaN and SiC technologies may
provide significantly improved power output. Reliability in spaceneeds to be investigated. Power added efficiency (PAE) and thethermal properties of the devices need to be improved if cooling isdifficult in space. Also important is a phase-locked power combining
scheme at the device, circuit and quasi-optical levels. Semiconductor
spatial-power-combining oscillator arrays are addressed for the SPS.10
The oscillator arrays consist of optical-link distribution networks,
phase-correction loops, and extended resonance oscillator arrays. The
beam-scanning capabilities are investigated.

Fig. 3.2.1 Average RF output power versus frequency for various
electronic devices11 and semiconductors.12
Trew 12 reviewed state-of-the-art microwave semiconductor
technology. The current state of the art of microwave solid-state
devices and microwave tubes is shown in Fig. 3.2.1. GaAs
metal-semiconductor FETs (MESFETs), GsAs pseudomorphic highelectron mobility transistors (PHEMTs), and SiC MESFET have thebest of semiconductors. The output of a single solid-state device is lessthan 100W, but higher output power can be obtained through124

power-combining and phased-array technology. Recent improvementsin the growth of wide bandgap semiconductor materials, such as SiC
and the GaN-based alloys, provide the opportunity to design andfabricate microwave transistors that demonstrate performancepreviously available only from microwave tubes. Microwave power
amplifiers fabricated from 4H-SiC MESFETs and AlGaN/GaN HFETs
have an RF power performance, particularly at an elevated temperature,
superior to comparable components fabricated from GaAs MESFETs
or Si transistors.
3.2.2 Microwave vacuum tubes
The average RF output power versus frequency for various
electronic devices is shown in Fig. 3.2.1. Some of the SPS designs are
based on a microwave power transmitter with microwave tubes such as
klystrons and traveling wave tubes (TWTs). The electronic tubes arecharacterized by their high efficiency (>70%) and high power output(normally of the order from several hundreds of watts to kW). Since
the output power is a few to 10 W per element in the typical cases, thehigh power from one electronic tube must be distributed to individual
antennas via power dividers. A recent analysis shows that the vacuum
tube devices are superior to semiconductor devices for microwave
power transmission system in terms of the weight per unit power,
DC-to-microwave conversion efficiency, and total cost. Magnetrons
have many advantages compared with other microwave devices. The
output power of magnetrons can be much smaller than that of klystrons
adopted in the NASA SPS reference system. Magnetrons are
inexpensive because they are mass-produced for microwave ovens.
They were, however, not entirely suitable for application to the SPS
because they lack frequency and phase stabilities essential for the
microwave beam control.
A new concept of a microwave transmitter, called a phase controlled
magnetron (PCM),13 satisfying both requirements of high efficiency
and beam controllability, has been developed. A new phased-arraysystem with the PCMs is also proposed for the SPS. The
injection-locking technique and phase-locked loop (PLL) feedback bycontrolling an anode current are used for the PCM. The PCM canstabilize and control the frequency and phase of the microwave. A
phased array with a phase and amplitude controlled magnetron(PACM) has recently been proposed.14 The PACM is developed withan injection locking technique and PLL feedback to an anode current
125for frequency and phase control and to an external coil current
controlling the magnetic field for amplitude control.
3.2.3 Phase shifters and power dividers
Microwave technology development is essential to the MPT.
Examples of microwave devices to be developed are listed below.
1) High-efficiency power transmitters with low harmonics
Very low harmonics and spurious emissions are required to avoid
interference with communications.
2) Low-loss phase shifters
Drive currents to control PIN diodes used in phase shifters are notnegligible since the order of power losses of the diodes is comparable
to the power treated.
3) Low-loss power dividers
Losses are especially high in high-power stages.
3.2.4
Microwave devices, circuits, and systems(1) Power generation devices and circuits
Many advanced solid state devices have recently been developedand improved. For instance, wide-bandgap devices such as GaN havesignificant power outputs particularly at relatively low microwave
frequencies of 2.4 and 5.8 GHz. Linearity and efficiency are always
desired not only for these devices but also for many other applications.
From the viewpoint of manufacturing huge quantities required for SPS,
III-V based devices have disadvantages over Si-based devices, simply
because III-V compounds are not abundant or cheap. Associatedcircuit technologies such as high efficiency amplifiers need to beadvanced while keeping their linearity. This is a challenge even forconventional communication and radar applications but is particularly
relevant to SPS where the total power is huge and loss abatement inorder to decrease the heat generation in space is a problem. Powercombining schemes have also been investigated. To date, however,no convincing results practical to the MPT have been realized.
It is important to seek alternative solutions such as vacuum tube
technology, while keeping the efficiency, linearity and reliability issues
in mind.
(2) Beam Control
It is important that all power should be harvested in the MPT as
opposed to enhancement of the antenna gain for the specific target in126the case of conventional phased-array systems. Pointing of the beam
is another important issue. In the past, retrodirective methods basedon the phase conjugation and digital control as well as combination
thereof has been investigated. These technologies should receive
more attention particularly in connection with total SPS architecture
and control aspects.
(3) Power recovery on the ground
Rectennas based on efficient diodes have been the primary vehicle
for this subject. Although DC conversion efficiency exceeding 80%
has been demonstrated in the laboratory environment, the large-scalefeasibility has not been tested. In addition, there may still exist someroom for improvement in both circuit technology and device
technology. It should be investigated if there exist any other power
recovery methods with potentially superior overall system efficiency.
Rectennas consist of antennas and rectifiers for transforming
microwave to direct current (DC). The development of a highly
efficient system is essential. The power densities in the receiving area
are not constant. How to combine rectenna elements with different
output powers is also important for the total efficiency. Harmonic
emissions generated by the nonlinearity of the diode rectifiers should
be minimized.
(4) Emerging Technologies
There may be possibilities to use new material and device
development such as carbon nanotubes and other exotic materials
including artificial or metamaterials for new devices and components.
Circuit architecture and technology require a new look from the MPT
point of view rather than traditional microwave communication andsensor applications.
Issues include high power, large-scale
deployment, manufacturability, efficiency, linearity, reliability andcontrollability.
(5) Active Integrated Antenna (AIA )There are unique developments for the SPS from the microwave
point of view as distinguished from the ordinary use of the microwave
technology such as telecommunications. These three points include
1) pureness in spectrum, 2) high-power and high-efficiency power
generation and highly efficient detectors in small, light packages, and
3) precise beam control for a large phased-array antenna combining a127huge number of subarrays.
To cope with the second requirement for the microwave technology,
the large plate model with a layered sandwich configuration was
proposed. The advantage of this configuration is the effective
integration with DC power generation, microwave circuit operation
and radiation, and their control. The Active Integrated Antenna (AIA)
technique is also considered as a promising microwave technology.
The AIA is defined as a single entity consisting of an integrated circuit
and a planar antenna. The AIA has many features applicable to the SPS.
The small, thin, light and multifunctional AIA can realize a powertransmission part of the spacetenna (space antenna) with a thin
structure.
The AIA is roughly categorized into two types: the cavity type andthe array type (Fig. 3.2.2). The cavity type AIA consists of a resonantcavity and a gain medium plate composed of a solid-state device and aradiator. In this case, the gain medium is made with a high-density
active device alignment resulting in the high power generation bymode locking. In the array type AIA, periodic alignment of the active
device and the radiator for producing the selected mode is adopted.
As a result, the AIA is made more compact by removing the resonantcavity. By using these natures, many applications of the AIA havebeen proposed.
For instance, a 2x2 AIA panel array wasdemonstrated in Fig. 3.2.3.15(a) Cavity type(b) Array typeFig. 3.2.2 Category of AIA.16

128

Fig. 3.2.3 The 2x2 AIA panel array.15

3.3 Beam Control
High transmission efficiency is essential for the SPS. In order toattain this efficiency, a huge antenna system is required. Phase
synchronization among units becomes important since the system is
too huge to distribute a reference signal to all the units. A retrodirective
system is used in transmitting power in the direction of an arriving
pilot signal. Precise beam control, low sidelobes, and low grating lobes
are necessary in this system. Effects of amplitude and phase errors
should also be taken into account.
3.3.1 Transmission efficiency
If all elements of an array are equally excited, this is simple and thegain factor and antenna efficiency become maximal, but the main beam
covers only a part of the total energy. For higher transmission
efficiency, a rectenna site must cover the first sidelobe including thefirst null. This needs more area and is inefficient since the extended
area includes a weak region. Sidelobe levels decrease slowly as afunction of a distance from the center, and sidelobes have higher
energy and could interfere with nearby communications. When the
distributions on the transmitter aperture are Gaussian (Gaussian taper),a high transmission efficiency is obtained. The fact that the output
power is maximal at the center of the transmitting array, however,causes the following problems. The output power of the center
amplifiers is the maximum and heat dissipation is quite difficult.
Although different kinds of outputs for the Gaussian distribution arenecessary in the array, their efficiencies should be high. The same129

effects as the tapering could be obtained under the equal excitation if
phases of the elements are adjusted properly through optimization
methods such as genetic algorithms and particle swarm optimization.17
Three kinds of antenna configurations for the SPS, array of small
radiators, array of apertures, and a single aperture, are presented andtheir realization is compared.18
3.3.2 Phase synchronization among units
An SPS system is so huge that the array is composed of many units.
‘Unit’ is defined as the area where the surface of the array is assumed
to be flat from the view point of antenna with an accuracy of less than
0.1 wavelength. The system is too large (a kilometer) to distribute areference signal generated by a single reference oscillator. Althoughbeam steering is possible in each unit, a beam radiated from a unit
could cancel that of another one depending on the phases of theirreference signals. Although the reference signal could be distributed bycables, optical fibers, or radio, it would be difficult to use it as a phasereference for an oscillator because the cable length and phase shiftinside each circuit must be taken into account. If independent
oscillators are used, their requirements for stability and accuracy would
be quite strict. Although their frequency can be synchronized, it would
be difficult to adjust their phases. Some studies on the phaseadjustment have been conducted.19
3.3.3 Retrodirectivity
A rectenna site sends a pilot signal to the SPS in order to indicate its
position. The SPS system sends a power beam back to the rectenna site.
This is called retrodirectivity, in which a signal is reflect back in the
direction of the incoming signal as described in section 2.3.5. A signal
from each antenna element (radio frequency; RF) is mixed with a local
oscillator (LO), and the intermediate frequency (IF) signal istransmitted back through the antenna element in the direction of thesender. The lower sideband of the output of the mixer is a phaseconjugation of the received signal as shown below.20

VIF = VRF cos(ω RF t + θ n ) × VLO cos(ω LO t )1= VRF V LO{cos((ω LO −ω RF )t −θ n) + cos((ω LO + ω RF )t +θ n)}2(3.3.1)The sign of the phase part θn of the lower sideband, ωLO −ωRF, is
130

opposite to that of the input, ωRF. For safety, any coherent beam should
not be transmitted if no pilot signal exists for safety. The number of
hardware systems is the same as that of antenna elements, totaling
more than several hundred million in the SPS. It would be unrealistic
to apply this system to all elements. Let’s call this the hardware
retrodirective system. The number of receiving elements is the same asthat of transmitting elements and is huge in the SPS. Calibration ofeach element would be quite difficult.
A transmitter and frequency doubler are demonstrated withhigh-efficiency AlGaN/GaN based on a HEMT active integrated
antenna (AIA).21 Their application to retrodirective arrays based on
the above principle and their use for beam-control applications to the
SPS are also presented. The transmitting frequency is equal to the pilot
frequency, ωRF if ωLO =2ωRF.
A retrodirective control transmitter was developed anddemonstrated.22 A subarray is composed of 63 5.8 GHz-microstrip
patch transmitter antennas and a 2.9 GHz antenna. Two subarrays areused for the demonstration. The phase conjugation was attained
through a digital signal processor at down-converted 10 MHz signals.
3.3.4 Software retrodirective system
The retrodirectivity can be attained by measuring the direction ofarrival (DOA) with fewer elements and setting the direction of thebeam of the transmitting array to the DOA. This method is lessexpensive than hardware methods and various beam-forming methods
can be applied. We call this the software retrodirective system. Thissystem forms beams using optimization methods like genetic
algorithms and particle swarm17 in order to obtain lower sidelobes,
high transmission efficiency, and multiple beam formation. Thismethod is expected to be feasible in the SPS as the required responsetime for the change of arrival directions can be slower thancommunication applications.
A new beam control system for SPS is proposed and demonstrated.23
This system is a kind of software retrodirective system and uses a
spread-spectrum pilot signal for the DOA estimation. The same
frequency is used for power transmission and a carrier of the pilot
signal and the DOA is measured under the transmission. Band-passfilters and software synchronization are used.

131

3.3.5 Effects of antenna array amplitude and phase errors
Accurate beam control is necessary for maximizing energy transfer
to the Earth and limiting radiation in the undesired directions so thatexisting telecommunication systems are not adversely affected. The
center of the microwave beam should stay confined to within 0.0005
degrees of the center of the rectenna. A retrodirective system will be
used for this severe requirement. The beam control accuracy of the
SPS-MPT system will be achieved by a huge number of thepower-transmitting antenna elements. The beam control accuracy is
proportional to phase errors of each subarray and N-3/2, where N is thenumber of subarrays. 24 N depends on the total diameter of thetransmitting antenna and spacing of subarrays. The phase errors must
include errors in target detection, structure distortion errors, and errorsof phase shifters. In order for an SPS-MPT system to realize the 0.0005
degree beam control, the total phase errors in the system must be
suppressed to a few degrees. These technologies are under research atpresent.25 It is noted that the beam collection efficiency is as importantas the beam control accuracy. The beam collection efficiency depends
on the sidelobes and the grating lobes. Amplitude and phase errors ofan antenna array do not cause significant errors on the direction of themain beam if these errors are not too large, but they can increase in thesidelobe level and decrease the transmission efficiency. They are notdesirable for the SPS.
3.3.6 Current antenna technology and future forecast.
The antenna specifications for the SPS are still quite demanding.
The best solution would be a hybrid design, combining the array
technology and deployable reflector antenna technology. In thisapproach, we may also use a sparse array concept by filling the largest
aperture with smallest number of elements. An antenna system with areconfigurable beam-steering reflector is proposed.26
Optimization methods such as genetic algorithms and particle
swarm17 can be used in the optimal array configuration. Currently,
mesh reflector antennas operating at the desired frequencies with up to
35-meter diameter are feasible. Additionally, due to the narrow band
operation of the design, one may also consider membrane deployed
Fresnel reflector antennas with much large diameters. These
reflector/Fresnel antennas can be used as the elements of the large
sparse array. Many issues need to be addressed including array132

topology, VLBI type operation among the elements, stability and selfcorrecting, and power handling.

3.4 Rectennas
A rectenna is composed of a rectifier (diode) and an antenna. A highefficiency in the power conversion is essential. A typical experimental
satellite will transit 100-400 kW of 5.8 GHz microwave from 10-20m
square phased array antenna in a low-Earth orbit to the ground, and arectenna will receive less than 1mW of microwave power. Highefficiency rectennas have been developed to cope with such low power
are developed. Suppression of harmonics caused by the nonlinearity ofdiodes in a rectenna should also be studied as soon as possible.
Low-power rectennas can be applied for ubiquitous power sources (seeSec. 3.6) and the RF-ID (radio frequency identification).27
In a large rectenna array used for the SPS, the power density of the
microwave beam is not constant over the entire array. The output of alarge rectenna array composed of some rectenna panels depends ontheir connections because outputs of the panels are not the same. It isthus important to optimize the total output.28 The power density in therectenna site used for the SPS system has to be kept low forenvironmental health and safety. It is necessary to develop a highefficiency rectenna for low power.
It is important to use a diode with a low turn-on voltage for highefficiency rectennas. To rectify multiple octaves, a different approachfrom standard matching techniques was used. In a rectenna application,
the antenna itself can be used as the matching mechanism instead ofusing a transmission-line matching circuit.29
The sum of the outputs of two or three rectenna panels depends onwhether they are connected in parallel or in series or in a hybrid
configuration under the same microwave circumstances. Conclusions28
based on the experiments are: (1) The sum of the dc outputs from two
rectenna panels connected in parallel is larger than that from thoseconnected in series. (2) The sum of the dc outputs of two rectenna
panels is generally smaller than the sum of the dc outputs of theindividual panels unless their outputs are equal. (3) A higher output ofdc power from a rectenna array can be achieved when the arrayelements are carefully balanced. Based on experiments, an optimum
method is proposed for connection of individual rectenna elements toform a high-efficiency rectenna array. Obtained is a similar
conclusion30 that it is effective to divide the entire aperture by smaller133apertures or smaller panels inverse-proportionally to the power density
Characteristics of almost all past rectennas are for rectifying over
100mW, and the RF-DC conversion efficiency is less than 20% at
1mW microwave input. Kyoto University recently developed a new
mW-class rectenna. All circuit parameters were surveyed for a higher
efficiency at 1mW microwave input, and the efficiency of a newrectenna at 1mW is approximately 50%. The new rectenna iscomposed of a printed-circuit dipole, an LPF and a rectifying circuit ona coplanar line whose impedance is 200Ω.

3.5 Measurement and Calibration
Measurement and calibration are important issues for the SPS and
MPT. Some problems and test programs for the SPS system aredescribed below.
3.5.1 Huge antenna array measurements on the ground and in
space
Microwave measurements are necessary for evaluation of power,interference, spurious emissions, etc. The SPS uses a huge antenna
array on the order of kilometers. Its radiation pattern and output power
must be evaluated before launch. The system requires retrodirectivity.
Correct measurement of the arrival direction of the pilot signal isessential as well as precise beam steering with an accuracy of less than
100 m for the SPS in GEO at a distance of 36,000 km.
3.5.2 Rectenna measurements
Transmitters and rectennas radiate harmonics because of thenonlinearity of rectifiers used in them. A new method of measuringharmonics has been proposed.31
3.5.3 Self calibration of antenna gain and phase errors
Errors of antenna elements result in errors in the direction of arrival(DOA) estimation of a pilot signal and in the beam formation in thetransmission. It would be important to make a self-calibration system
for a large number of elements in the array. A very long baseline
interferometry (VLBI) system for radio astronomy uses known stars as
references for its calibration.

1343.5.4SPS Antenna Test Program
3.5.4.1 General Considerations.
Space is a harsh environment with large temperature gradients, highsolar wind, and strong ionizing radiation. It is possible to approximate
such physical conditions only in restricted laboratory environments.
The testing of large antennas presents not only the usual difficulty ofmaking accurate RF measurements on a substantial aperture, but also
the unusual problems of devising tests that accurately predict theperformance impact of the harsh mechanical and thermal conditions of
the operating environment of the antenna.
The large assembly of panel arrays that constitutes the SPS antenna
must be tested at each construction level for both electromagnetic andenvironmental performance. The following are building blocks of theantenna. 1) The basic radiating element, 2) A subpanel of size large
enough to test the radiation pattern performance of the basic element in
the array environment. This subpanel may be formed by assembling a
few rows and columns of basic radiators. 3) An antenna panel, that is
the array of radiating elements that is repeated to form the SPS antenna,
4) The whole SPS antenna.
The test program consists of radiation measurements and datacollection to evaluate by computer extrapolation the performance of
the SPS antenna in the extraterrestrial environment.
3.5.4.2 SPS Antenna Element Test Program.
The basic antenna element can be tested both for radiation andenvironmental performance in conditions close to those of space.
If temperature gradients are expected between the front (radiatingside) and the back (feed side) of the element, these environmental
temperature conditions should be reproduced as closely as possible
during the radiation test program.
The radiation patterns, the gain and the feed matching conditions
over the operational frequency band can be evaluated by the usualmeans of an antenna test site and a network or antenna analyzer. The
antenna should be placed over a ground plane of dimension close to thesubarray discussed in the next section.
Of interest is the evolution of the antenna element performance withaging in space. The ionizing radiation, the temperature gradients andthe particle bombardment change the metal and dielectric properties of
the construction materials. Some materials may have been used forspace application and historical data may exist on their aging. Whether
135novel materials are employed or not, it is suggested that an accelerated
aging test program be performed on the radiating element and theantenna performance be evaluated at various stages of the aging test to
establish the limits of acceptable performance.
With the highly automated instrumentation available from a variety
of suppliers it is possible to prescribe a very detailed test set of gain
and matching conditions for the antenna element. Over the bandwidthof the radiator 100 match and boresight absolute gain measurements
should be collected for initial performance evaluation. Hemisphericalradiation maps (contour plots) for polarization and cross polarization
do not need to be recorded with such repetition. Only five recordings
over the operating band should suffice.
A much more detailed test program can be established as the element
design nears completion, depending on the materials and the geometryof the radiator.
3.5.4.3 SPS Antenna Subarray Test Program.
This program evaluates, under extraterrestrial environmental
conditions, the performance of the radiating elements within the array
and investigates the effects of aging on their gain, pattern and matchingconditions. This step is necessary because it may not be possible torealistically test the performance of an entire panel in simulated space
ambient conditions. If such testing is possible, then the subarray test
program could be skipped. However, it is good engineering practice tosimulate the effects of mutual coupling between the array elements
without building an entire array antenna.
The mutual coupling between array elements can substantiallychange the performance of the single radiator in terms of matching andpattern performance. With aging of materials, the coupling conditions
can give rise to substantial reflections at the element aperture andsurface waves over a dielectric layer covering the antenna. Most effects
can be predicted by computer simulation, but a test program is an
insurance of later performance.
Without the specific element physical design, it is possible to give
only some general rules for the construction of a subarray. The
strongest intra-element coupling happens in the plane of theE-polarization, the weakest in the H-plane. It is possible to reproduce
array conditions for an element by constructing a subpanel of 11 rows
and five columns of radiators. Only the element at the center of the
subpanel needs to be excited, all the others can be match-terminated.
136

As in the case of the single element program, the tests evaluate theeffects of the space environment on the radiation and matchingconditions of the subpanel gather data for confirming the computer
simulation of the SPS array performance.
Space ambient temperature gradients should be recreated during thetests to investigate their effects on possible warping of the radiating
elements and their mechanical support structure. The adverse effects ofaging of the array materials with temperature changes, ionizing andparticulate radiation can be detected during this test program. Subtle
changes in the array element pattern point to degradation of side lobe
performance. Changes in gain, polarization and matching conditions(return loss) should be carefully analyzed for the possible degradationof the SPS antenna.
With computerized data collection, absolute boresight gain andreturn loss measurement losses can be performed at 100 frequencies
equally spaced over the SPS antenna band.
As in the case of the single isolated element, hemispherical radiation
maps can be collected for the E-field polarization and thecross-polarization.
3.5.4.4 SPS Panel Test Program
The SPS antenna is formed by assembling a number of arrays. The
physical size of these antennas is large and the costs of simulating thetemperature gradients in space over the extent of the array may beprohibitive. From the subarray test program data, the maximum
possible mechanical misalignment of the array elements can becomputer simulated and may not need to be tested. The array tests
consist of gain measurements, radiation patterns and radiation maps
(contour plots of the sidelobes). The contour plots may not be difficult
to measure if the panel is too large to be tested with an antenna
positioner in an open range.
Given the size of the SPS antenna panels, the measurement of gain
and radiation patterns are most conveniently performed in a compact
range.32 Following the methods given in,32 it is possible to compensate
for many of the measurement errors arising in a compact range. A
spherical or a planar compact range can be used for this set ofmeasurements
The evaluation of the far out sidelobes is always somewhat
compromised in a compact range. A well designed open range is thebest means to collect contour plots of an antenna’s far sidelobes. The
137

antenna must be mounted on a two- or three-axis
positioner. It is advisable to reduce the size of a panel so it can betested on an open range, for two reasons.
1) Far sidelobes can be mapped with accuracy.
2) It is necessary to test the effects of panel warping and spacing on
the far out sidelobes.
Although computer simulations can predict with some precision theeffects of geometrical changes on the sidelobes, it is always a goodengineering practice to test the accuracy of the computations. A largeantenna positioner (e.g. from Scientific Atlanta) can orient with great
accuracy ( few milliradians) a large and heavy structure up to 100 kg. It
is suggested that a support structure be built for at least two panels of
the SPS and the effects of panel separation and warping on gain andsidelobe performance be tested experimentally.
The data collected in this test phase make it possible to constructrealistic computer models for the performance of the SPS antenna.
These considerations point out the advisability of building an
ensemble of panels as large as feasible for testing in a compact and an
open range.
3.5.4.5 SPS Antenna Test.
The antenna is expected to be so large that it cannot be tested in its
entirety on the ground. Computer simulations can give a sufficiently
close evaluation of expected antenna performance in terms of gain,
beamwidth and near sidelobes.
The antenna can be tested with some accuracy once in orbit. A seriesof gain and pattern tests can be performed with receivers on airplanes
or a group of RF receivers located in predetermined positions on the
ground. These measurements are performed to substantiate thecomputer simulations performed in the previous phase of the tests.
These measurements should be performed with reduced RF powerfrom the SPS until it has been tested that the beam is positioned on thecollector-receiver and that no strong sidelobes radiate in unwantedareas.

3.6 Spin-off technologies
The wireless power transmission (WPT) technologies including
MPT are not only applied to the SPS, but also have applications on the
ground. The WPT and wireless communication technology are138

fundamentally the same technology. Maxwell’s equations govern both.
The difference is the viewpoint of waves (Fig. 3.6.1). For wirelesscommunication, a radio wave is used only as a carrier of information.
The WPT system uses the carrier of a radio wave. Therefore, almost alltransmitted power must be received and only a very narrow band isnecessary. Efficiency is very important for the WPT system. The power
densities are different. The WPT uses many orders of magnitude higher
power density than communications. The energy carrying wave is inprinciple a monochromatic wave without modulation.
The advantages of the WPT over conventional power transmission
using stationary conducting lines are as follows.
・ As WPT does not need any power lines connecting a power
generator and power consumers, it has more freedom of choice of bothtransmitter and receiver locations. Even mobile transmitters andreceivers can be chosen for the WPT system.
・ One transmitter site can distribute power toward multiple
customers simultaneously like broadcasting.
・ A receiver called a rectenna is lighter than a commonly usedbatteries or photovoltaic cells. Furthermore, power is available atrectenna sites as long as the WPT is operating. This removes the worry
of a power shortage due to battery exhaustion.
Wireless Communication

Wireless Power Transmission
Information

Transmitted
Power
Receiver→Information
Transmitter

Time and Space

Receiver→PowerElectric Power
To User
Only Carrier for WPT

Frequency
Wide

Transmitted
PowerTime andSpaceTransmitted
PowerPower Source

Transmitted PowerPower
SourceFrequency

Receiver→Information

・We need only a part of transmitted
power (because an information is
amplified)・Wide band because we transmit aninformation

Transmitter Very Narrow
・We need almost all transmitted
power
・Very narrow band because weuse carrier (pure)・Efficiency is most important.

・Efficiency is not essential.

Fig. 3.6.1 Difference between wireless communication and wirelesspower transmission. ©RISH, Kyoto University
139

The power loss of WPT is much less than that of line transmission
although this depends on antenna aperture sizes. The power loss inpropagation is less than 1% even for a long distance transmission ofseveral tens of thousands kilometers like used for the SPS.
One application is the MPT for moving targets, e.g., a fuel-free
airplane, a fuel-free electric vehicle (EV), and moving robot in alimited area. In the late 1980's, Canada proposed a program to develop
a long-endurance, high-altitude platform called SHARP (Stationary
High Altitude Relay Platform).33 The idea is to float an unmanned
light-weight airplane for a long period, circling at an altitude of about
21 km for the purpose of relaying radio communications signals over awide area. To maintain the platform airborne for weeks or months,
fuel-less airplanes powered by microwave energy transmitted from theground were proposed and investigated by experiments in Canada andin Japan. The Japanese experiment was called MILAX (MIcrowave
Lifted Airplane eXperiment).34 In Japan, MPT to an electric vehicle(EV) was also proposed and the MPT experiment was carried out withsmall scale functional model of the EV.
Another application of the MPT is ground-to-ground (G-to-G) powertransmission without wires toward a distant place where wired power
distribution networks are either unavailable or with very poor
distribution. Two experiments were carried out, one in the USA in
1975 and one in Japan in 1995-96. In the Japanese experiment,
fundamental data on microwave power transmission were collectedunder various weather conditions and experiments on the connection of
the rectennas were carried out. The merits of the G-to-G MPT arequick installation and easy disassembly because there are no lines
between the transmitter and the receivers. The MPT can thus be used
as an emergency power supply.
The most recently proposed MPT application is “Ubiquitous Power
Source (UPS)” or “Wireless Power Source.” In a ubiquitous powersource, where power is fed via microwave, one can extract electric
power from weak microwaves anywhere and at any time. Laboratory
experiments have already been carried out in a shielded room.35 In the
UPS concept, the use of the microwave is very similar to that in thecommunication system. The emitted microwave expands and weak
microwave power is received in many receivers.
A recent system similar to UPS is RF-ID (radio frequency
identification). The RF-ID is based on a chip. This chip carries140

information that can be retrieved through radio waves. The power usedin the chip is also provided by the radio waves. The most common
application of the RF-ID is a verification system. This is also called an“IC tag,” and is receiving attention all over the world, in the form ofstandardization and research. The rectenna technology can be applied
to the rectifier of the RF-ID.
Presently, most RF-ID research assumes the 915 MHz band. RF-ID
is still in the development phase. If energy exchange throughmicrowaves becomes necessary for RF-ID, rectennas would benecessary. However, to our knowledge, only communication needs
are being studied. Hitachi has come up with a micro-chip operatingnear 2.45GHz.36 It is a super-miniature RF-ID chip, called μ chip. It
has dimensions of 0.4mm×0.4mm×0.06mm, and is being pursued sothat it can be inserted into a sheet of paper. The rectenna part of a μ
chip is shown in Figs. 3.6.2 and 3.6.3.
Table 3.6.1 RF-ID and frequencies37
Frequency
MethodDistance
Cost
Applications

120-150kHz
Electromagnetic
induction
〜50cm
fair

13.56MHzElectromagnet
ic induction
〜1m
good

Immobilizer (cartheft prevention)
Livestock control

IC cardBaggagecontrol

915MHzMicrowa
ve
〜5m
Very
good

2.45GHzMicrowave〜1m
excellent
μ chipSandwich-type antenna
Enlargement
strip

IC chip with terminals
on two sides
Thin-type antenna

Fig. 3.6.2 μ-chip antenna38

Fig. 3.6.3 μ-chip rectifier38

141

1

W. C. Brown, Beamed microwave power transmission and its application to
space, IEEE Trans. Microwave Theory Tech., vol. 40, no. 6, 1239-1250,
1992.
2
G. Goubau and F. Schwering, On the guided propagation of electromagnetic
wave beams, IRE Trans. Antennas and Propagation, AP-9, pp. 248-256,
1961.
3 Similar results are obtained in Uno, T. and S. Adachi, A design ofmicrowave wireless power transmission by the aperture illumination of
maximum transmission efficiency (in Japanese), IEICE Trans., vol. J66-B,
no. 8, 1013-1018, 1983.
4
Kraus, J.D., Antennas (2nd ed.), McGraw Hill, 1988.
5This is based on a Response to the Question ITU-R 210/1 on wireless powertransmission in ITU by the following contribution; JAXA, Present status of
wireless power transmission toward space experiments, Document No.
1A/53-E, ITU Radiocommunication study groups, October 1, 2004.
6
Each item corresponds to that of the Question ITU-R210/1.
7
T. Hatsuda, K. Ueno., and M. Inoue, Solar power satellite interference
assessment, IEEE Microwave Magazine, vol. 3, no. 4, 65-70, December,
2002.
8
Applications and Characteristics of Wireless Power Transmission,
Document No. 1A/18-E, Task Group ITU-R WP1A, Reference Question
210/1, ITU Radiocommunication Study Group, October 9, 2000.
9
Matsumoto, H., Microwave power transmission from space and related
nonlinear plasma effects, Radio Science Bulletin, no.273, pp.11-35, June
1995.
10
J. Choi and A. Mortazawi, Free-space power-combining oscillator array forsolar power transmission, The Radio Science Bulletin, no. 311, 47-53, 2004.
11
V. L. Granatstein, P. K. Parker, and C. M. Armstrong, “Scanning theTechnology: Vacuum Electronics at the Dawn of the Twenty-First Century,”
Proc. IEEE, vol. 87, pp. 702–716, May 1999
12
R. J. Trew, SiC and GaN Transistors—Is There One Winner for Microwave
Power Applications?, Proc. IEEE, vol. 90, no. 6, 1032-1047, June 2002.
13
Naoki Shinohara, Hiroshi Matsumoto, and Kozo Hashimoto, Solar power
station/satellite (SPS) with phase controlled magnetrons, IEICE Trans.
Electron., E86-C, 1550-1555, 2003.
14
Naoki Shinohara and Hiroshi Matsumoto, Design of Space Solar Power
System (SSPS) with Phase and Amplitude Controlled Magnetron, Proc. of2004 Asia-Pacific Radio Science Conference, pp.624-626, 2004.
15
Shigeo Kawasaki, A unit plate of a thin, multilayered active integrated
antenna for a space solar power system, The Radio Science Bulletin, No.310,
pp.15-22, 2004.
16
S. Kawasaki, A Patch-Plate Array Type of Active Integrated Array Antenna142

for SPS2000, ISAS Bulletin, 43, pp. 47−55, 2001.
Y. Rahmat-Samii, et al, Particle Swarm Optimization (PSO): A novel
paradigm for antenna designs, The Radio Science Bulletin, no. 305, 14-22,
2003.
18
T. Takano, A. Sugawara, and S. Sasaki, System considerations of onboard
antennas for SPS, The Radio Science Bulletin, no. 311, 16-20, 2004.
19
e.g., R. Chernoff, R., "Large Active Retrodirective Arrays for Space
Applications," IEEE Trans. on Antennas & Propagation, Vol. AP-27, No. 4,
pp. 489-496, July 1979.
20
R. Y. Miyamoto and T. Itoh, Retrodirective arrays for wirelesscommunications, IEEE Microwave Magazine, vol. 3, no. 1, 71-79, March
2002.
21
K.M.K.H. Leong, et al., Active antenna approach for power transmission.
The Radio Science Bulletin, no. 311, 21-30, 04.
22
F. E. Little, et al., Development of a retrodirective control transmitter for
wireless power transmission, ibid, 38-46, 2004.
23
K. Hashimoto, et al, Space solar power system beam control withspread-spectrum pilot signals, ibid, 31-37, 2004.
24
R. J. Mailloux, Phased array antenna handbook, Artech House, 1994.
25
N. Shinohara, Y. Hisada, M. Mort, and JAXA SSPS WG4 Team, Request
and Roadmap for Microwave Power Transmission System of Space Solar
Power System (SSPS), Proc. of IAF2005, Japan, 2005.
26
Varadan, V.K., J. Xie, K.J. Vinoy, and H. Yoon, Nano- and Micro-Devices
for Performance lmprovement of Space Solar Power System, The Radio
Science Bulletin, no. 310, 36-46, 2004.
27
See, 3.6 Spin-off technologies.
28
Shinohara, N. and H. Matsumoto, Dependence of dc Output of a Rectenna
Array on the Method of Interconnection of Its Array Elements, ElectricalEngineering in Japan, 125, 9-17, 1998.
29
J. A. Hagerty et al., Recycling Ambient Microwave Energy with
Broadband Rectenna Arrays, IEEE Trans. on Microwave Theory andTechniques, Vol. 52, Issue 3, pp. 1014 – 1024, March 2004.
30
Murao, Y., and T. Takano, An Investigation on the Design of aTransmission Antennas and a Rectenna with Arrayed Apertures for
Microwave Power Transmission, Electronics and Communications in Japan,
vol.83, no.2, 1-9, 2000.
31
Y. Fujino and M. Fujita, An experimental study of re-radiation of higher
harmonic waves from a rectenna for microwave power reception, IEEJ Trans.
Vol. 117-A, No. 5, 490-495, 1997 (In Japanese).
32
D. A. Leatherwood and E.B Joy, Plane Wave, Pattern Subtraction, Range
Compensation, IEEE Trans. on Antenna and Propagation, Vol. 49, No. 12,
December 2001, pp 1843-1851.
17

143

33

J. J. Schlesak, A. Alden and T. Ohno, A microwave powered high altitude
platform, IEEE MTT-S Int. Symp. Digest, 283-286, 1988.
34
H. Matsumoto, et al., MILAX Airplane Experiment and Model Airplane,
12th ISAS Space Energy Symposium, Tokyo, Japan, March 1993
35
N. Shinohara, T. Mitani, and H. Matsumoto, Study on Ubiquitous Power
Source with Microwave Power Transmission, C07, Proc. of URSI GA, India,
2005.
36
M. Usami and M. Ohki, The μ-chip: an ultra-small 2,45 GHz RFID chip for
ubiquitous recognition applications, IETCE Trans, Electronics, vol. E86-C,
no. 4, 521-528, 2003.
37
SSK seminar, RFID business (in Japanese), January 23, 2004.
38 M. Usami, An Ultra Small RFID Chip: μ-chip (in Japanese), Microwave
Workshop MWE2003, WS09-01, Yokohama, Japan, Nov. 2003.

144

Chapter 4

Influence and Effects of SPS

This chapter describes the SPS interaction with space and theatmosphere, compatibility with communications and radio astronomy,
and influence of the SPS and MPT on human health and bio-effects;
it also summarizes the pros and cons of the SPS. To assure
environmental safety and health, the proposed limit of the maximum
power at the center of the microwave transmission beam should becontrolled by tightly tuned phased-array techniques and by automatic
beam defocusing.

4.1 Interaction with space and the atmosphere
4.1.1

Atmospheric effects

Fig. 4.1.1 Rain rate (mm/h) exceeded for 0.01% of the average year
(Rec. ITU-R P.837-4).1
Very few groups have worked on possible effects of microwaves
on the atmosphere. Studies presently available refer to potential
145effects via heating of the ionospheric electrons or via ionization of
the air. Observations of transient luminous events (sprites, blue jets,
elves, ...) in the upper atmosphere set basic questions on theelectrical processes that develop in the Earth environment. It isclear that new studies are needed on all phenomena that may
influence the atmospheric electrical conductivity and thus the global
electric circuit.
Heating of the ionospheric electron population may affect theionospheric plasma and the atmosphere in different ways (see section
4.1.2). The effects are probably more important between 100 and 250
km where the main chemical process controlling the ionosphericplasma concentration is the electronic recombination of O2+ and NO+.
They obviously depend on the level of enhancement in the electron
temperature.
The rain attenuation can be calculated based on ITU-R PN618.2 Incase of Tokyo, the specific attenuation γR = 0.2dB at 5.8GHz since its
rain rate is 50mm/h for 0.01% of the time (about 52.5 minutes peryear). Rain rates for a part of Europe are shown in Fig. 4.1.1. Since
the effective path length Le = 4.5km, the value of attenuation A=0.9
dB (81%). However, γR = 0.1dB and A= 0.45 dB (90%) at 5 GHz.
These values are smaller in Europe due to less rain as shown in the
figure.
The microwave SPS beam is scattered by rain or hail.3 For a rain
rate, R = 50 (mm/hour), frequency of 2.45 GHz, and the elevation
angle of 47 degrees, the attenuation is about 0.015 dB/km.
Furthermore, the maximum interference intensity P (W) received byan antenna for terrestrial radio relay links near the rectenna site for a
power density of 23 mW/cm2 is
P = 6.7×10-11R1.4h,
where h (m) is the scatterer length. If R = 50~150 mm/hour and h =3~10 km, then P = 0.1~1 mW. This level, however, would not cause
nonlinear problems and interference can be removed by filters.
Batanov et al4 studied the effects of powerful microwaves on theatmosphere have been studied both theoretically and experimentally.
Of particular interest are the experimental studies devoted tocleaning the troposphere of ozone. The idea involves artificiallyionizing the air using high power electromagnetic waves. The
necessary threshold field strength and intensity are 680 kV/m and 6 x
104 W/cm2 at 15 km, corresponding to a 6 GHz continuous wave.
Since microwave pulses are used for the excitation of discharges, this146

breakdown electric field level can be several times higher. Althoughboth levels are much higher than the values that will be achieved
from the SPS, one sees that microwave radiation may have positiveeffects on the Earth environment. We also need to study and monitor
potential negative consequences.
4.1.2 Ionospheric effects
Although much more published works are available, there are noconclusive observations or propagation models to provide adefinitive view about the effects of microwave radiations on theionosphere.
(1) Ohmic heating
The first obvious effect of high power microwaves on theionosphere is resistive or Ohmic heating. The absorption of the radio
waves can be calculated from the electron density andelectron-neutral collision frequency profile. The effect is largest in
the lower ionosphere (D and E regions) where the collision
frequency is highest. Although the effect is expected to be small withincreasing frequency, it could still be significant. Several authors5
have calculated the heating effect of 3 GHz waves. They estimate
that, for a power density of about 16 mW/cm2, the electron
temperature could increase from about 200 K in the E region to about1000 K. A temperature increase would result in a decrease of electron
density because of a decrease in the temperature-dependent
recombination rate of O2+ and NO2+. In the D region an increase in
the attachment rate to O2+ also reduces electron density. To ourknowledge no measurements of electron heating from high power
microwaves in the ionosphere exist. The reason is probably two-fold:
the difficulty of measuring electron temperatures on short time scales
in the D region, and the lack of microwave heating experiments.
Even if VHF and UHF radars of sufficient power-aperture produce
heating effects, it is difficult to use them as both heating andmeasuring devices. It should also be noted that the heating effects
may not be well represented by Maxwellian electron distributionfunction analysis6 that is often assumed in analysis of incoherent
scatter radar data, so that standard analysis techniques may not beapplicable.
Microwave injections from a rocket have been tried 7 (andpresented in a poster review8), but Ohmic heating effects could not147be observed. The lack of measured microwave heating in theionosphere should not caste doubt on the reality of Ohmic heating
caused by powerful microwaves, but only points out the
shortcomings of the attempts made so far to measure it. On thisrocket flight, the expected heating effect was less than 100K, which
was below the detection limit of the Langmuir probe. However, theilluminated plasma volume was very small.7
Because the ionospheric heating efficiency varies as the inverse
square of the radio frequency, heating effects equivalent to thosefrom high-power microwaves can be achieved at much lower powers
by heating at a lower frequency. This is done using ionosphericmodification or heating facilities that are simply high-power(~1MW) short-wave (2 to 10 MHz) transmitters radiating upwards
using high gain (16 to 30 dB) antenna arrays. D-region Ohmic
heating effects are clearly observable indirectly through theconductivity and current modulation experiments 9 and thesometimes dramatic heating effects on polar mesospheric summer
radar echoes. 10 , 11 Direct measurements of the temperatureenhancement using incoherent scatter radar are, however, difficult
and rare.12(2) Self-focusing effects
Thermal self-focusing takes place as a result of a positive feedback
loop. Small natural density fluctuations give rise to a spatial variationin the refractive index, resulting in slight focusing and defocusing of
the microwave. This slight differential heating of the ionosphericplasma results in a temperature gradient driving the plasma from thefocused region and thereby amplifying the initial perturbation. Sucheffects are well known and have been studied from HF-heating
experiments, but it is unclear how important this is for an underdense
plasma where the microwave frequency is much greater than theplasma frequency.
(3) Three-wave interactions
The heating effects discussed above are the result of non-resonantinteractions with the plasma. Another effect of high power
microwaves is the production of plasma waves through resonantinteractions, in particular through parametric instabilities. There havebeen several theoretical predictions that microwaves at high power
may produce instabilities in the ionosphere. Matsumoto 13 and148

Matsumoto et al.7 demonstrated that the microwaves may decay into
forward-traveling electron plasma waves (Raman scattering) or ion
acoustic waves (Brillouin scattering) and a backward-travelingsecondary microwave. The electron plasma waves could be
Langmuir waves when the excitation is parallel to the geomagnetic
field, or electron cyclotron waves for excitation perpendicular to thefield. Dysthe et al.14 and Cerisier et al.15 examined the case of two
powerful microwaves having a frequency difference equal to the
local ionospheric plasma frequency, typically 2 to 10 MHz. The
ponderomotive force, which is proportional to the product of the twoelectric fields, can be strong enough to excite a parametric instability
that results in Langmuir waves being produced. One result of aground-based radar experiment near 1 GHz16 shows that such effects
may indeed take place in the ionosphere. The three-wave interactions
are expected to be most effective in the F region, above about 170
km.
Apart from the radar experiment of Lavergnat et al.,16 there is, toour knowledge, only one other report of plasma waves being causedin the ionosphere by powerful microwave transmissions. This wasfrom a 830W, 2.45 GHz transmitter on a mother-daughter Japaneserocket experiment (MINIX) where electrostatic electron-cyclotron
waves at 3/2 the local electron gyrofrequency and electron plasma
waves above the local plasma frequency were observed 17 ,7 andpresented in a poster review.8 It was found that the excited waves
differed from the initial theoretical expectations18 in that the line
spectrum expected from a simple three-wave coupling theory was infact a broad spectrum, and the electron cyclotron harmonics werestronger than the Langmuir waves. Both these features could be
successfully modeled using a more realistic computer simulation19
where the nonlinear feedback processes were fully incorporated.
From these simulation results, it was estimated that 0.01 percent of
the microwave beam energy from the SPS would be converted toelectrostatic waves.
In conclusion, there have not been enough experiments withpowerful microwaves in the ionosphere to determine with confidence
the importance of instabilities as a loss mechanism for the beam andas a source of plasma waves and heating of the ionosphere. In theneighborhood of the satellite the power density will be high and its
effects on the ionosphere will be examined experimentally. Caremust be taken in the choice of frequency separations if multiple
149

frequencies are used to beam down the power. Effects on theatmosphere are not expected.
4.1.3 Effects of electric propulsion on the magnetosphere
In the process of SPS construction, large high-power electric
propulsion systems are needed. The electric propulsion systems
inject heavy ions accelerated by electrodes powered by thephotovoltaic cells. For transformation of orbits around the equator,
the heavy ions are injected perpendicular to the Earth’s magnetic
field. The injection can strongly disturb the electromagnetic
environment surrounding the ion engine in the plasma sphere and themagnetosphere through interaction between the heavy-ion beam andthe magnetosheric plasmas. The interaction between the heavy-ion
beam and the magnetic field has been studied theoretically. 20 , 21
Based on an MHD analysis, Chiu20 predicted that Argon ion injection
could excite Alfven waves propagating along the magnetic field
down to the ionosphere and being reflected back. He also predictedthat injected Argon ions can accumulate in the magnetosphere,
significantly changing the plasma environment.
Curtis andGrebowsky21 showed that the bulk of the injected ion beam is not
stopped in the magnetosphere. However, the relatively small
fraction of the beam that is not stopped may give rise to a large
distortion in the magnetospheric plasma population. They also
evaluated possible loss mechanisms from the magnetosphere for thisartificial ion component.
The interaction of the heavy ions and the surrounding magnetizedplasma field has been studied by particle simulations using hybrid
code, where motions of ions are solved as particles while electrons
are treated as a neutralizing fluid. As an initial response to theinjection, a shock structure can be formed in the ambient plasmaalong with generation of magneto-hydro-magnetic waves andassociated heating of the background plasmas.22
It has to be noted that heating processes and parametric instabilities
may also take place within the plasmasphere, in the neighborhood of
the satellite. The plasma is less dense but there is a high level ofwave activity. The artificial generation or loss of extremely low(ELF) and ultra low (ULF) frequency waves in that region may haveconsequences on the dynamics of the radiation belts150

4.2 Compatibility with other radio services andapplications
Undesired emissions, such as grating lobes, sidelobes, carrier
noise, harmonics, spurious, and out-of-band emissions of any Space
Solar Power System must be suppressed sufficiently to avoid
interference with other radio services and applications, in accordancewith the provisions of the ITU-R Radio Regulations (RR). Thisapplies not only to any eventual full power operational systems, butalso to all developmental, test and intermediate power prototype
systems, both in space and on the ground. Hence this is a near termissue, even though it may take decades before full systems become
operational.
Most SPS microwave systems are assumed to use frequency bands
around 2.5 GHz or 5.8 GHz. These are allocated in the ITU-R Radio
Regulations to a number of radio services and are also designated for
Industry, Science and Medical (ISM) applications.
The ITU Radio Regulations define ISM applications as follows.
RR 1.15 Industrial, scientific and medical (ISM) applications (ofradio frequency energy): Operation of equipment or appliances
designed to generate and use locally radio frequency energy forindustrial, scientific, medical, domestic or similar purposes,
excluding applications in the field of telecommunications.
Note that as presently defined the ISM bands are for local use
only.
The following Radio Regulations govern the use of the ISM
applications.
RR 5.150 The following bands: 13,553-13,567 kHz, 26,957-27,283
kHz, 40.66-40.70 MHz, 902-928 MHz in Region 2, 2,400-2,500
MHz, 5,725-5,875 MHz, and 24-24.25 GHz are also designated forindustrial,
scientificandmedical(ISM)
applications.
Radiocommunication services operating within these bands must
accept harmful interference which may be caused by theseapplications. ISM equipment operating in these bands is subject tothe provisions of No. 15.13.
RR 15.13 Administrations shall take all practicable and necessary
steps to ensure that radiation from equipment used for industrial,
scientific and medical applications is minimal and that, outside thebands designated for use by this equipment, radiation from such
equipment is at a level that does not cause harmful interference to a151radiocommunication service and, in particular, to a radionavigationor any other safety service operating in accordance with theprovisions of these Regulations.
The intended bandwidth of SPS emissions is quite narrow, as an
essentially monochromatic wave without modulation will be used.
As noted in Section 4.1.2, care must be taken in the choice of
frequency separations if multiple frequencies are used to beam down
the power.
4.2.1 Compatibility with other services such as Radio
Astronomy
An interference assessment on mainly 2.45 GHz was published inIEEE Microwave Magazine. 23 The following is a partial list of
mechanisms by which an SPS could cause interference.24
1) The power transmission signal, its harmonics, and any sidebands
that might be present in the fundamental frequency reference, which
will appear coherently at all power amplifiers of the system.
2) Noise generated in the power output stages. This will not be
coherent at individual power amplifiers and so will not be beamed
like the power signal but spread much more widely in angle. The
spectrum might only be a few tens of MHz wide if the transmitterelements are highly tuned (e.g. klystrons) and could be broader forsolid-state devices.
3) Thermal noise emitted by the solar collector arrays. This may
represent a significant broadband component of radiated power inthe microwave spectrum.
4) Reflection by the collector arrays of high-powered transmitters inspace or on the ground. This can occur over a wide range of
frequencies.
5) Spurious emission at unwanted frequencies or in unwanted
directions from the power transmitters associated with component
failure of the amplifiers themselves or of parts of the antenna
system. It is not certain whether failing transmitters might become
unlocked in frequency, generate spurious modes or just die quietly.
6) Harmonics and noise generated in the rectenna.
7) Intermodulation between the power signal and other radio signals
generated in a rectenna or in nonlinear elements in the high fieldareas near a rectenna.
Carrier noise, harmonics, and spurious emissions of the WPT152

signal must be quite small to avoid interference with other radio
services in operation around the world. Grating lobes and sidelobesof the WPT beam should be low enough to make the affected region
as small as possible. Also, grating lobes should be mitigated because
they are a direct loss of transmitter power.
It is important that compatibility be evaluated for full systems, notjust single units. Many units of 1 GW will be required if satellite
solar power is to represent a significant contribution to future energy
needs.
The 2.45 and 5.8 GHz ISM bands share a common frequency
allocation worldwide. Familiar applications include radio controllers,
microwave ovens, RF-ID (radio tags) and drying of cut lumber. To
date they have been used for WPT applications for demonstrationand experimental purposes.25
The 2.45 GHz ISM band (2400-2500MHz) and the 5.8 GHz ISMband (5725-5875 MHz), however, have already been allocated tovarious other services as well. Recently, the 2.45 GHz ISM band has
been widely used for Radio LAN (IEEE 802.11b and g) applications.
The frequency allocation of the 2.45 GHz Radio LANs occupies
almost the whole band.
The 5.8 GHz ISM band is also heavily used for various
applications. The 5725-5850 MHz band is allocated to theRadiolocation
service.
DSRC(Dedicated
Short-Range
Communications), described in Recommendation ITU-R M.1543, isalso expected to use the band. The 5850-5925 MHz band is allocatedto Fixed/Mobile services and is used for terrestrial Electronic News
Gathering (ENG) in some countries.
The second, the sixth, ninth and 20th harmonics of the 2.45 GHz
ISM band overlap with radio astronomy bands (4.9-5.0, 22.1-22.5
and 48.96-49.06 GHz). It is expected that the interference level near
4.9 GHz would be very much higher (40 dB or more, depending on
the system) than the harmful interference threshold. Hence the upper
half, 2.45-2.5 GHz, cannot be used for SSPS.
The harmonic situation is better for the 5.8 GHz band. However,many harmonics of the 2.45 GHz and the 5.8 GHz band overlap the
76 - 116 GHz radio astronomy band.
The spurious and out-of-band (OOB) emission from high-powertransmitters is likely to interfere with adjacent radio astronomy bands.
Frequency allocation for SPS must avoid harmful interference withradio astronomy applications, which use very sensitive passive
153

receivers. Spurious emissions must be suppressed sufficiently toprotect the Radio Astronomy Service, and rectennas must be located
far from radio astronomy observing sites. As radio astronomy is fully
passive and celestial objects have no lower limit in intensity ofemission, its observing systems have been advanced to become
extremely sensitive.
4.2.2 Reflection and Thermal Emission from Solar Cells26,24
In the radio region, solar cells reflect solar radio emission in acontinuous frequency region of 100 MHz through 100GHz andbeyond. The power flux densities are 0.1 - 1 M×10-26 W/m2 /Hz(quiet Sun) and 100 - 1000 M×10-26 W/m2 /Hz (burst).
These values are six to ten orders of magnitude higher than thosefrom typical cosmic radio sources. This means that radio
astronomical observations may be affected by these reflections,depending on the telescope location on the Earth.
The apparent angular size of the solar cell array with a diameter ofabout 13 km is close to 1 arcminute, which is 1/30 of those of the
Sun and the Moon, and a little larger than Jupiter (about 40
arcseconds), the largest planet of the solar system. Because the SPS
systems are always seen at the same locations, they would prevent
astronomical observations of those regions of the sky. Even the
JAXA 2002 Model, for example, whose diameter of the primary
mirror is about 3km (about 15 arc seconds in angular diameter),would obscure many celestial objects forever. This model systemdoes not represent a large addition to the terrestrial power capability.
The effect grows as multiple such systems are added to make an
operational system.
Optical and infrared astronomy will suffer from reflection by thesolar cells. This was studied extensively for the earlier US system.
The National Research Council panel concluded: 27 "The diffuse
night-sky brightness produced by the reference SPS would interfere
seriously with optical astronomical measurements from the Earth.
This interference would be concentrated in an area on either side of
the satellite arc and would prevent the measurement of weak
astronomical objects in those areas."
An important effect of the SPS on radio astronomy arises from thepassive thermal radiation of the solar cells. There will be zones
centered on the geostationary orbit in which observations over a wide
range of frequencies will be precluded, not just at harmonics of the154power transmission. For example, in the studies of the US reference
system proposed in 1978, the thermal radiation from the collectors of
one satellite were estimated to produce a level about 10 dB below the
detrimental threshold levels of Recommendation ITU-R RA.769
assuming a unity gain antenna over a wide range of
frequencies. The interference is detrimental if one points a radio
telescope so that receiving sidelobes above 10 dBi lie on theorbit. For 32-25log(φ) sidelobes, that means pointing closer thanabout ±7.5 degrees to the orbit. There will also be noise generated
in the transmitting tubes or power transistors, which could be rather
wide in bandwidth if these power amplifiers are not narrow-band
devices. This could be stronger than the thermal noise, but will
depend on the characteristics of the particular devices used.
With a full system of satellites in orbit, satellites would be
distributed fairly continuously around the GEO, so that at any radioor optical observatory a band of sky centered on the orbit would bepermanently blocked from certain observations at essentially all
frequencies. The substantial loss of observable sky resulting fromsuch wideband noise emission would be severely harmful.
Such effects are, however, expected to be smaller. For example, inthe case of the JAXA 2003 model shown in Fig. 2.4.9, the reflected
light by the huge mirror is specific and directed to the solar cellpanels and the light reflected by the cells is directed perpendicular tothe direction of the Earth.

4.3 MPT on Human health and bio-effects
The concept of solar-power satellites (SPS) and wireless-powertransmission (WPT) envisions the generation of electric power bysolar energy in space for use on Earth.28,29 The system would involve
placing a constellation of solar power satellites in geostationary
Earth orbits. Each satellite would provide between 1 and 6 GW ofpower to the ground, using a 2.45 or 5.8-GHz microwave beam (seeTable 2.3.2). The power-receiving rectenna on the ground would be a
structure measuring 1.0 to 3.4 km in diameter. The higher (5.8 GHz)
frequency has been proposed since it has a similar atmospherictransparency. Although, in principle, the higher frequency could
involve a reduced size for the transmitting and receiving antennas, it
can be seen from the table that current designs have opted for larger
transmitting antennas and smaller rectenna sites, but a larger power
density on the ground to conserve land use, especially in Japan.
155

A joint effort between the Department of Energy (DOE) and the
National Aerospace Administration (NASA) in the US extensively
investigated the feasibility of SPS-WPT during 1976-1980. The
effort generated a Reference System Concept for Solar Power
Satellites. The DOE–NASA Reference System involved placing a
constellation of solar power satellites (5 x 10 x 0.5 km deep) in
geostationary Earth orbits, each of which would provide 5-GW ofpower to major cities on the ground, using a 2.45-GHz microwave
beam. The Reference System’s sixty satellites were contemplated todeliver a total of 300 GW of generating capacity. The transmitting
antenna was about 1 km in diameter. The power-receiving rectennaon the ground was a 10 x 13-km structure.
Japan's Ministry of Economy, Trade and Industry (METI) had
announced plans to launch research for a solar-power-generation
satellite and to begin operating a giant solar-power station by 2040.
This program is expected to design and operate an SPS-WPT systemthat would ensure the microwaves would not interrupt cellular
mobile telephone and other wireless telecommunications services.
The Japan Aerospace Exploration Agency (JAXA) has proposed andevaluated various system configurations for operation at 5.8 GHz(see Table). For example, the JAXA2 model would have amaximum power density of 100 mW/cm2 (1000 W/m2) on the ground.
A smaller transmitting system would have a density of 26 mW/cm2(260 W/m2) at the rectenna site on the ground.
A variety of environmental considerations and safety-related
factors continue to receive consideration, albeit at a low priority level.
The biological effects and health implications of microwave
radiation have been a subject of study for many years.30,31,32 In fact,
the cumulative data have allowed the establishment ofrecommendations for safety levels for humans under a variety of
exposure conditions. For example, the ICNIRP (and Japanese)guideline is 5 or 1 mW/cm2 for occupationally exposed vs. thegeneral public, at either 2.45 or 5.8 GHz. 33 Although thecorresponding limits for IEEE standards for maximum permissible
human exposure to microwave radiation, at 2.45 or 5.8 GHz, are 8.16
or 10 mW/cm2 averaged over six min, and 1.63 or 3.87 mW/cm2
averaged over 30 min, respectively, for controlled and uncontrolledenvironments,34 the IEEE standards have recently been changed tothe similar one to that of the ICNIRP. 35 The controlled anduncontrolled situations are distinguished by whether the exposure156

takes place with or without knowledge of the exposed individual, andis normally interpreted to mean individuals who are occupationally
exposed to the microwave radiation, as contrasted with the generalpublic.
As can be seen from the Table, the proposed power densities range
from 23 to 180 mW/cm2 above the rectenna at the center of the
microwave beam, where power densities would be maximum. At
2.45 GHz, the power density is projected to be 1 mW/cm2 at theperimeter of the rectenna. Beyond the perimeter of the rectenna or
15 km, the side lobe peaks would be less than 0.01 mW/cm2.
Clearly, beyond the perimeter of the rectenna, the potential exposure
would be well below that currently permissible for the generalpublic.
The danger of loss of control of highly focused beams may beminimized by tightly tuned phased-array techniques and byautomatic beam defocusing to disperse the power if loss of control
occurs. Defocusing would degrade the beam toward a moreisotropic radiation pattern, which would give rise to even lower
power density on the ground.36
Near the center of the microwave beam, power densities would begreater than the permissible level of exposure for controlled
situations. Except for maintenance personnel, human exposure would
normally not be allowed at this location. In the case ofoccupationally required presence, protective measures, such as
glasses, gloves and garments could be used to reduce the exposure toa permissible level.
However, at 25 mW/cm2, research has shown that some birds
exhibit evidence of detecting the microwave radiation. This suggeststhat migratory birds, flying above the rectenna, might suffer
disruption of their flying paths. Moreover, at higher ambient
temperatures, larger birds tend to experience more heat stress thansmaller ones, during 30 min of exposure.37 This result is consistent
with the knowledge that the larger birds, having a larger body mass,
absorb a relatively greater quantity of microwave radiation than dothe smaller birds. The additional heat, from microwave energy
deposited inside the body, could be stressing the thermal regulatory
capacity of the larger birds. Thus to assure environmental healthand safety, the proposed limit for the "center-of-beam" power
densities is approximately 25 mW/cm2 for microwave transmission.
Note that the average absorption remains fairly stable for frequencies
157

above 2 GHz,30,38 except when the frequency becomes much higher,
i.e., 10 GHz, where the skin effect takes over, the maximum
tolerable exposure at 5.8 GHz would be essentially the same as for2.45 GHz.
We have to discuss the microwave (over GHz) effect on humanhealth imposed by the SPS system. There is a long history
concerning the safety of microwave energy.39
Contemporary RF/microwave standards are based on the results ofcritical evaluations and interpretations of the relevant scientific
literature. The specific absorption rate (SAR) threshold for the mostsensitive effect considered potentially harmful to humans, regardlessof the nature of the interaction mechanism, is used as the basis of thestandard. The SAR is only related to a heating problem, which isregarded as the only microwave effect on human health.
Discussions about the maximum microwave power density inside
the rectenna site are necessary. The maximum power density
depends on the antenna size and the frequency, which directly affectthe total cost. In the present JAXA2 model, the microwave power
density is 100 mW/cm2 at the center of the rectenna site, which isabove the safe level. This area should be strictly controlled. Outsideof the rectenna area, the intensities are kept below the safe level. A
possible change of the safe level in the future could cause changes of
the SPS design.

4.4 Precautionary Principle
Any new technology has to face the "Precautionary principle." Asshown in several papers, 40 the way to apply the Precautionary
principle has considerably evolved. Guidelines for applications havebeen recommended by several administrative bodies. They are
explicitly aimed at risk management. Political decisions are taken
according to risks identified at a given time and to the research
activities that must be pursued or undertaken. As far as theapplication of the SPS technology is concerned, where the main risks
are environmental, it must be clear that:- the present expertise must be used to identify and prioritize therisks, and to define critical parameters to continuously monitor in the
preparation phase as well as in the operation phase, and- complementary research programs must be set up.
If there is a recommendation from the URSI council on SPS, it will
have to include the concept of the Precautionary Principle.
158

4.5 Summary of Pros and Cons of SPS
Arguments for promoting SPS projects have recently been
re-activated due to increasing interest in clean energy,. Opinions infavor of the SPS concept are based on experiment data and feasible
technological arguments, while those against or suspicious of the
SPS concept arise from concerns about unknown factors that might
be harmful to human life or about possible strong interference withradio astronomy and existing telecommunication networks. The
arguments are categorized and listed in the following two subsections.
In sections 4.5.1-3, the number after ‘See #’ means that of the items
listed in section 4.5.4. Pros and Cons of SPS in Q&A.
4.5.1 Pros of SPS• SPS is the cleanest base-load power that can substitute for fossil
fuels. [See #1.]
• SPS is one of promising solutions for the global warming
problem.
• Sustainable energy sources include wind, solar power,
geo-thermals and biomass. SPS is a clean energy source that does
not generate CO2 once in operation. It is recognized the only powersource that can be a base-load supplier for 24 hours among various
non-atomic clean power sources. [See (2)]• SPS is believed to contribute to reducing CO2 emission since SPS
does not generate CO2 under operation. [See 2.1.2.]• The incoming energy transmitted by SPS from space to the Earthis five orders of magnitude less than the total solar radiation
reaching the Earth. Therefore, SPS does not worsen global
warming problems. [See #8.]
• Research for microwave technologies to realize the low cost SPS
should be continued.
• SPS and radio astronomy may not be completely compatible onsome frequency ranges, but this is left as a choice based on futurepriority of sustainability of human society.
4.5.2 Cons of SPS(1) Economics and Environment• Construction of a huge space structure usually is expensive andtakes a long time. [See 2.1.4.]
159

• SPS is not compatible with nor superior to current electric powersources in terms of cost. [See 2.1.4.]• The space environment (ionosphere, magnetosphere and theozone layer) may be damaged when the SPS power beam passes
through it. [See #4, 5, 6.](2) Launch and Transportation• One worries about orbit congestion and interference with thecurrent communication satellites. Therefore, suitable room on GEO
can not be given for a huge space construction like SPS. [See #12.]• It is hard to build a huge space structure in a short period.
• Space debris may seriously damage the SPS. [See 2.3.1.]• Some believed that the energy (and the CO2 production) required
to build, launch, and transport SPS will exceed the energy
generated as a power source. [See #11.]• The construction cost may be rather high if we use the current
launching system. [See 2.1.4.]• High construction cost and long term project result in hugebudget. [See 2.1.4.](3) Safety and Compatibility• SPS microwaves might be hazardous for human beings. [See #15,
16.]
• Birds in flight are subject to monochromatic microwaves. [See#13.]
• Radiation by SPS is not compatible with radio astronomical
research. [See 4.2.2.]• SPS may interfere with other electric equipments.
• Unexpected and harmful radiation resulting from malfunctions of
the SPS operation might interfere with existing telecommunications
networks and airplanes. [See e.g. #16.]• SPS should follow ITU regulations for radio astronomy
observation and other uses of the spectrum, which may limit its
practical application. [See #7, 18.]
4.5.3 Other issues of SPS• Is SPS really required, and if so, when should it be introduced toour society?
• For public acceptance, the exposure level of the microwave
160

density must be less than the safe level set by the government basedon scientific research. Currently, most of the microwave power
beam is designed so that the microwave density is less than 1
mW/cm2 at the edge of the rectenna on the ground. Problems due
to malfunctions and exposure of people on Earth and in airplanes
can be avoided by highly reliable technologies.
• To operate SPS safely, precise and reliable beam control forhigh-power beams might be achieved with a pilot signal from the
Earth. [See 4.2.1.]• SPS should be safe for flying vehicles such as airplanes. [See #3.]
• Insects and plants should not live under rectenna. [See #14.]• The rectenna site should be located so as to minimize interferencewith the existing telecommunications systems. [See 4.2.1.]• The SPS subsystems are launched to low-earth orbit (LEO) by
reusable launch vehicle (RLV), where some of the subsystems arejoined and checked their basic operation. These subsystems aretransported to GEO by electric propulsion orbital transfer vehicle(EOTV). It is not clear that such a complicated construction
process should be employed and the largest parts of the solar cellpanels may well be carried directly to GEO. [See 2.3.1.]• Scientific findings from radio astronomy contribute to thefoundation of new technologies. Telecommunications systems are
essential for our daily life. Therefore SPS research should consider
compatibility with them. [See 4.2]
Investigation Themes• To show the correct economic estimation based on higher
reliability• Compatibility
with
the
radio
astronomy
andthetelecommunications• Observation of change in the layers surrounding the Earthimposed by high-power beams.
4.5.4 Pros and Cons of SPS in Q&A
Presently, there are many pros and cons for SPS. Some may
disappear rapidly, while others may arise in the future. Here areanswers to the most frequently asked questions based on present
knowledge.
• General
1. Is SPS more sustainable and cleaner energy than solar power?
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Yes. Base-load power is necessary for sustainable energy. Terrestrial
solar power is good for intermittent use, but it is not appropriate forbase-load power. SPS can be used as a base load. The land under
the rectenna would be available for agriculture. See, 2.1.3.
• Economics and Environment
2. Is SPS expensive?
Power generation cost could be competitive or cheaper than otherenergy sources. This will require reduction of launch costs by verylarge factors. Innovative technologies, especially radio wave
technologies, have to be developed in order to reduce the SPS cost.
See, 2.1.4.
3. Is SPS hazardous to airplanes?
Yes, the power density at the SPS beam center is above theregulation of US Federal Aviation Administration (FAA). This area
should be placed off limits to air traffic control.
4. Does SPS affect shortwave propagation in the ionosphere?
Nonlinear effects in the ionosphere at the SPS microwave intensities
are expected in the center of the beam only. If shortwave
propagation is affected it would be limited to that region. See 4.1.2.
5. Does SPS affect the atmospheric chemistry, the ozone layer, andmore generally on the climate?
There would be no effects. See 4.1.1.
6. Will ion engines destroy the ozone layer?
Hardly. Hydrogen chloride used by solid rockets could destroy theozone layer. However, SPS will be transferred by Reusable Launch
Vehicles (RLVs) in stead of solid rockets. RLVs use liquidhydrogen and oxygen. Their destruction of the ozone layer is
expected to be very small.
7. Does SPS change the night sky?
Angular size of the solar cells or mirrors assumed in the recent SPS
models is smaller than that of Jupiter. The reflected direction islimited since the mirrors cause an almost specular reflection. See
4.2. If a huge number of SPS are orbited, more effects would occur.
8. Does sending energy from outside the Earth worsen global
warming?
No. The power density of the beam is weaker than that of sunlight.
The total energy used on the Earth is only 1/7000 of the amount of
the solar energy reaching the Earth. Since the efficiency of rectenna
is quite high, the amount of heat radiation is very small and no CO2
is emitted.
162

9. Will SPS become huge debris?
Present usage in GEO is to move satellites above GEO by several
hundred km following end of life. Most parts of SPS, however,should be used multiple times in further SPS generations. The rest
should be brought back to the Earth.
10. Could the microwave beams become weapons?
No. The maximum power density can never exceed the designed
level. Microwave weapons will use high-power pulses at shortranges. Its design is quite different from SPS design. See 4.3.
11. Is more energy used to construct SPS than is obtainable from it?
No. The period to recover the energy for construction (energy
pay-back time) is less than one year.
• Launch and Transportation
12. There is no space in Geostationary Earth Orbits.
If spectrum congestion were negligible, satellites would only require
separation of about 64 km.41 This corresponds to 0.1° spacing. The
spectra of signal interference, however, has led regulators to
mandate orbital separations of from 1,280 to 2,560 km (2 to 4°
spacing) or more in order to avoid signal interference amongneighboring communications satellites using the same frequencies.
SPS could be operated with less separation. There have not yet
been adequate studies of the required separation between SPS andcommunication satellites.
• Safety and Compatibility
13. Are entire migratory flocks of birds cooked or at least
influenced en route?
They are never cooked. The reporting is generally for power
densities higher than the 23 mW/cm2 peak in the center of thebaseline SPS beam. It appears that the 23 mW/cm2 power density
did not disturb the birds much, but doubling that power density
would strongly affect the birds.42
14. Are insects affected by SPS?
No evidence has been found that 2.45 GHz continuous wave
microwaves at selected power densities from 1 to 50 mW/cm2 havebiological effects on honey bees.
15. Is SPS safe?
According to our present knowledge, yes. The power density of the
microwave beam is within the safe level at the perimeter of the163

rectenna site. See 4.3. Conservative government safety guidelines
allow microwave ovens to leak 5 mW/cm2. The allowable leaklevel in Japan is 1 mW/cm2 at 5 cm from the oven.
16. Is SPS beam harmful if directed in the wrong direction?
If the direction deviates from the rectenna site, the beam is defocusedand its intensity becomes very low. See 3.2.3. However,accidental or intentional redirection may be possible through thecontrol software.
17. Does SPS interfere with communications?
Probably. Needless to say, grating lobes and sidelobes of the MPT
beams and harmonics and spurious emissions should be suppressed
to avoid possible interference with communications. See 4.2.
18. Does SPS interfere with radio astronomy?
Probably. Undesired emissions of the MPT beams should be
suppressed sufficiently to avoid interference with radio astronomy
in accordance with the provisions of the ITU-R Radio Regulations.
See 4.2.
1

Recommendation ITU-R P.837-4, Characteristics of precipitation for propagation
modeling, ITU, 2003.
2 G. Maral and M. Bousquet, Satellite communications systems, 3rd Ed., John Wiley& Sons, 1993.
3 Furuhama, R., and S. Ito, Effects of high power microwave propagation tounionized armospere (in Japanese), Review of the Radio Research Laboratories,
Vol. 28, No. 148, 715-721, 1982.
4
Batanov, G. M., Batanov, I. A. Kossyi, and V. P. Silakov, Plasma Physics Reports,
Vol. 28, No. 3, 2002, pp. 204–228. (Translated from Fizika Plazmy, Vol. 28, No. 3,
2002, pp. 229–256.) Negative effects were first discussed by their group (G.A.
Askar’yan, G.M. Batanov, I.A. Kossyi, and A. Yu Kostinskii, Sov. J. Plasma Phys,
17(1), 48-55, January 1991) but they changed their idea from negative to positive.
5
Perkins, F. W. and R. G. Roble, Ionospheric heating by radio waves: Predictions
for Arecibo and the satellite power station, J. Geophys. Res., 83, A4, 1611-1624,
1978.
6
Stubbe, P., Modifying effects of a strong electromagnetic wave upon a weakly
ionized plasma: a kinetic description, Radio Sci., 16, 3, 417-425, 1981.
7
Matsumoto et al., Rocket experiment on non-linear interaction of high power
microwave energy beam with the ionosphere: Project MINIX toward the solar
power satellite, ISAS Space Energy Symposium, 69-76, 1982.
8
Rietveld, M. T., Ground and in situ excitation of waves in the ionospheric plasma,J. Atmos. Terr. Phys., 47, 12, 1283-1296, 1985 (review of poster paper presented at
URSI General Assembly, Florence, 1984)
9
Stubbe, P., H. Kopka, M. T. Rietveld, and R. L. Dowden, ELF and VLF wave
generation by modulated heating of the current carrying lower ionosphere, J.

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Atmos. Terr. Phys., 44, 12, 1123-1135, 1982.
Chilson, P. B., E. Belova, M. T. Rietveld, S. Kirkwood, U.-P. Hoppe, First
artificially induced modulation of PMSE using the EISCAT heating facility,
Geophys. Res. Lett., 27, 23, 3801-3804, 2000.
11
Havnes, O., C. La Hoz, L. I. Naesheim, M. T. Rietveld, First observations of the
PMSE overshoot effect and its use for investigating the conditions in the summer
mesosphere, Geophys Res. Lett. 30, 23, 2229, doi:10.1029/2003GL018429, 2003.
12
Kero, A., T. Bösinger, P. Pollari, E. Turunen and M. Rietveld, First EISCAT
measurement of electron-gas temperature in the artificially heated D-region
ionosphere, Ann. Geophysicae, 18, 9, 1210-1215, 2000.
13
Matsumoto, H., Numerical estimation of SPS microwave impact on ionosphericenvironment, Acta Astronautica, 9, 8, 493-497, 1982.
14
Dysthe, K. B.; Mjolhus, E.; Trulsen, J. Nonlinear mixing in the ionosphere,
Physica Scripta, 21, 122-128, 1980.
15
Cerisier, J. C., Lavergnat, J.; Rihouey, J. J.; Pellat, R. Generation of Langmuir
waves by nonlinear wave-wave interaction in the ionosphere, J. Geophys. Res., 86,
4731-4738, 1981.
16
Lavergnat, J., P. Bauer, J. Y. Delahaye, and R. Ney, Nonlinear sounding of theionospheric plasma, Geophys.Res. Lett., 4, 417-420, 1977.
17
Matsumoto, H., Microwave power transmission from space and related nonlinear
plasma effects, Radio Science Bulletin, no. 273, pp. 11-35, June, 1995.
18
Matsumoto, H., H. Hirata, Y. Hashino, N. Shinohara, Theoretical analysis ofnonlinear interaction of intense electromagnetic wave and plasma waves in theionosphere, Electronics and Communications in Japan, Part3, 78, 11, 104-114,
1995b.
19
Matsumoto, H., Y. Hashino, H. Yashiro, N. Shinohara, Computer Simulation on
Nonlinear interaction of Intense Microwaves with Space plasmas, Electronics andCommunications in Japan, Part3, 78, 11, 89-103, 1995a20
Chiu, Y. T., Fate of Argon ion injection in the magnetosphere, AIAA Paper
80-0891, 1980.
21
Curtis, S. A., and Grebowsky, J. M., Energetic ion beam magnetosphere injection
and solar powere satellite transport, J. Geophys. Res., Vol. 85, No. A4, 1729-1735,
1980.
22
Y. Omura T. Sakakima, H. Usui, and H. Matsumoto, Computer experiments on
interaction of heavy ion beam from a large-scale ion engine with magnetosphericplasma, IUGG 2003, Sapporo, (2003).
23
T. Hatsuda, K. Ueno, M. Inoue, Solar power satellite interference assessment,
IEEE Microwave Magazine, Vol. 3, No. 4, 65-70, December, 2002
24
A. R. Thompson, Effects of a satellite power system on ground-based radio andradar astronomy, Radio Science, 16, 35-45, 1981.
25
Present status of wireless power transmission toward space experiments,
Document No. 1A/53-E, Task Group ITU-R WPIA, Question 210/1, ITU
Radiocommunication Study Group, October 1, 2004.
26
Radiocommunication Study Group Document WP7D, submitted by Japan ,February 1996.
10

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27

Electric Power from Orbit, a Critique of a Satellite Power System, National
Academy Press, Washington, D.C. 198128
National Research Council, 2001, Laying the Foundation for Space Solar Power:
An Assessment of NASA's Space Solar Power Investment Strategy, National
Research Council, Washington, D.C.
29
Lin, J, "Space Solar-Power Stations, Wireless Power Transmissions, andBiological Implications", IEEE Microwave Magazine, pp. 36-42, March 2002.
30
Michaelson, S. and J.C. Lin, 1987, Biological Effects and Health Implications ofRadiofrequency Radiation, Plenum Press, New York.
31
Lin, J.C., "Biological Aspects of Mobile Communication Fields," Wireless
Networks, vol. 3, pp 439-453, 1997.
32
Lin, J.C., "Biological Effects of Microwave Radiation," In Electricity andMagnetism in Biology and Medicine. F. Bersani, ed., Kluwer/Plenum, New York,
pp 165-169, 1999.
33
ICNIRP, “Guidelines for Limiting Exposure to Time-varying Electric, Magnetic,and Electromagnetic Fields (Up to 300 GHz),” Health Physics, 74, 1998, pp.
494-522.
34
IEEE, Standard for Safety Levels with Respect to Human Exposure to Radio
Frequency Electromagnetic Fields, 3 kHz to 300 GHz, IEEE, New York, 1999.
35
Lin, J.C.. The New IEEE Standard for Human Exposure to Radio-Frequency
Radiation and the Current ICNIRP Guidelines, Radio Science Bulletin, No. 317,
61-63, June, 2006.
36
Osepchuk, J.M., Health and Safety Issues for Microwave Power Transmission,
Solar Energy, 56:53-60, 1996.
37
U.S. DOE, Proceedings of Solar Power Satellite Program Review, Office ofEnergy Research, Department of Energy, Washington, D.C., 1980.
38
Lin, J.C. and O.P. Gandhi, "Computer Methods for Predicting Field Intensity," inHandbook of Biological Effects of Electromagnetic Fields, (C. Polk and E. Postow,Eds.), CRC Press, Boca Raton, pp 337-402, 1996.
39
John M. Osepchuk and Ronald C. Petersen, “Historical Review of RF ExposureStandards and the International Committee on Electromagnetic Safety (ICES)”,
Bioelectromagnetics Supplement 6:S7-S16, 2003.
40
see
for
instance
Foster
et
al.,
2000,
http://www.biotech-info.net/science_and_PP.html
41
M.K. Macauley, Allocation of Orbit and Spectrum Resources for Regional
Communications: What's at Stake? Discussion Paper 98-10, Resources for the
future, December 1997 www.rff.org/Documents/RFF-DP-98-10.pdf
42
http://www.permanent.com/p-sps-bm.htm See also, R. Dickinson, "EstimatedAvian Temperature Rise During Flyover of a 5.8 GHz Wireless Power
Transmission Beamer," WPT-2001, La Reunion Island, France, May 14-17, 2001.

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Chapter 5

URSI and SPS

5.1 Technologies
The International Union of Radio Science (Union
Radio-Scientifique Internationale), a non-governmental andnon-profit organization under the International Council for Science,is responsible for stimulating and coordinating, on an international
basis, studies, research, applications, scientific exchange, andcommunication in the fields of radio science. Let us look at therelation of the SPS technologies with Terms of Reference of the
URSI Scientific Commissions listed in Appendix of the White Paper.
Various technical issues closely related to terms of specificCommissions are described below.
Commission D (Electronics and photonics) promotes research inelectronic devices, circuits, systems and applications, and it has abroad interest in SPS, particularly in MPT.
Substantial
technological areas covered by the Commission have direct
relevance to MPT. Many other emerging areas related to theCommission have significant potential for enhancing existing MPT
technology. More correctly, a task of Commission D scientists andengineers is to find the deficiencies of the existing technology and toenvision potential technologies to rectify the present short comings.
In addition, since Commission D is closely related to devices andhigh frequency component industries, manufacturability should be aimportant criteria in assessing the particular technology for MPT.
Since SPS requires huge investments even for the electronic parts,
the availability of particular materials and the manufacturability areof serious concerns.
Commission D scientists and engineers can play important roles in
microwave power generation, beam control and efficient power
recovery as well as related technologies on microwave devices,
circuits and systems. These activities are also related to antenna
technologies primarily addressed by Commission B.
Examples to be developed are highly efficient power transmitterswith low harmonics, low-loss phase shifters, and diodes for efficient
rectennas at low power.
The interest of Commission B (Fields and waves) is fields andwaves, encompassing theory, analysis, computation, experiments,and validation of fields and waves. One of the areas of emphasis is167antennas and radiation. A huge antenna array is essential for
microwave transmission from the SPS.
High transmission
efficiency (the ratio of the received power to the transmitted power)
is required. The suppression of grating lobes and side-lobes mustbe suppressed to avoid interference with communications and toguarantee bio-safety. This can be achieved by tapering the output
power distribution in the antenna array.
Microwave measurements and calibrations are necessary toevaluate power, interference, and spurious emissions from the SPS
and the rectennas. Contributions from Commission A(Electromagnetic metrology) are expected.
The Commission
promotes research and development of measurements and standards
in time and frequency, including infrared and optical frequencies, inthe time domain, in the frequency domain, in telecommunications,etc.
Direction finding and self-calibration systems should be developed
to calibrate the SPS antenna array with its huge number of elements.
These issues require signal processing techniques studied inCommission C (Radio communication systems and signal
processing), which promotes research and development in Signal andImage Processing in the area of radio science.

5.2 Environments
Atmospheric effects, including the ozonosphere, produced byand imposed on microwave beams, linear and nonlinear interactions
with the ionosphere and space plasma of the microwave beam shouldbe evaluated through theories, experiments and computer simulations.
(Commissions F, Wave propagation and remote sensing; G,
Ionospheric radio and propagation; and H, Waves in plasmas) In
SPS construction, a huge amount of materials must be transferredfrom Low-Earth Orbit (LEO) to Geostationary Earth Orbit (GEO) byelectric propulsion where accelerated ions are ejected from ion
engines. The interaction of the heavy ions with the surrounding
plasma could change the electromagnetic environment of themagnetosphere. These plasma processes is quantitatively evaluated(Commission H, which includes the areas of solar/planetary plasmainteractions).
Most SPS systems are assumed to use the frequencies of 2400 to2500 MHz or 5725 to 5875 MHz for MPT. Compatibility of SPS
with radio communications and radio astronomy are important issues
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for URSI. (Commission E, Electromagnetic noise and interference;and Commission J, Radio astronomy)
The evaluation of possible effects on human health and bio-effects
by microwaves transmitted from SPS is essential for public
acceptance. (Commission K, Electromagnetics in biology andmedicine)

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170

Chapter 6

Further Readings(1) A recent text book on SPS is P. E. Glaser, F. P. Davidson, K. I.
Csigi, Eds., Solar Power Satellites (Wiley-Praxis, New York,
1997). This is composed of five parts, The solar power satellite
concept Perceptions about energy for planet Earth civilization,
International SPS-related activities, Earth-based and space-based
infrastructure considerations, and SPS development, and covers awide range of topics on SPS useful for the general public.
http://www.praxis-publishing.co.uk/view.asp?id=64&search=home(2) A review by National Research Council is published in“Laying the Foundation for Space Solar Power: An Assessmentof NASA's Space Solar Power Investment Strategy” (National
Academy Press, Washington, D.C., 2001). The U.S. Congress
became interested in SSP and in FY 1999 appropriated funds forNASA to conduct the SSP Exploratory Research and Technology(SERT) program. The SERT program and its follow-on, the SSP
Research and Technology (SSP R&T) program, constitute the effort
assessedin
thisreport.
http://search.nap.edu/books/0309075971/html/(3) AP-RASC (Asia-pacific radio science) meeting was held inTokyo, Japan, in August 2001. This conference was sponsored by
the Japanese National Committee of URSI (the International Union
of Radio Science) and The Institute of Electronics, Information andCommunication Engineers (IEICE) in Japan and cosponsored by
URSI. A selection of papers presented at the Union Session wasentitled “Solar Power Satellite and Wireless Power Transmission,”were published in a special section of the IEEE Microwave
Magazine, vol. 3, no. 4, December 2002. The four papers cover
SPS research in Japan and the US, health and interference issues.
http://ieeexplore.ieee.org/xpl/tocresult.jsp?isYear=2002&isnumber=2
5789&Submit32=Go+To+Issue(4) A two-part Special Section on Space Solar Power Systems(SSPS) was published in the Radio Science Bulletin, nos. 310
and 311, in September and December 2004. The eleven papers inthe section are based on the invited talks at the 2003 Japan-UnitedStates Joint Work Shop on Space Solar Power Systems (JUSPS’03)
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held on July 3-4, 2003, at Kyoto University, Japan. These papers
cover an SPS demonstration experiment, microwave semiconductor,
tube, and nano- and micro-devices; passive and active antennas;
power combiners; and retrodirective systems. They are available
via the following URSI website.
http://www.ursi.org/RSBissues/RSBSept2004.PDF
http://www.ursi.org/RSBissues/RSBdecember2004.PDF(5) History of Microwave Power Transmission The following
papers are recommended for the understanding of MPT.
1. W. C. Brown, The history of power transmission by radio waves,IEEE Trans. Microwave Theory and Techniques, MTT-32,
pp.1230-1242, 1984.
2. W. C. Brown, The history of wireless power transmission, Solar
Energy, vol. 56, 3-21, 1996.
3. H. Matsumoto, Microwave power transmission from space andrelated nonlinear plasma effects, Radio Science Bulletin, no. 273,
pp. 11-35, June, 1995.
(6) Space Solar Power Workshop (http://www.sspi.gatech.edu/) is acontinuing conceptual design of how to build, finance, deliver,market, support, operate, and maintain an Space Solar Power
System for the world base load energy market. A book, "Silent
Power", summarizing the work of the SSPW to date will be
published here.
Some examples of SF (science fiction) books that treat SPS are as
follows.
(7) Ben Bova, Powersat, has been published in 2005. Twohundred thousand feet up, things go horribly wrong. An
experimental low-orbit spaceplane breaks up on reentry, falling toEarth over a trail hundreds of miles long. Its wake is the beginningof the most important mission in the history of space. America
needs energy, and Dan Randolph is determined to give it to them.
He dreams of an array of geosynchronous powersats, satellites thatgather solar energy and beam it to generators on Earth, freeing
America from its addiction to fossil fuels and breaking the power ofthe oil cartels forever. But the wreck of the spaceplane has left his
172company, Astro Manufacturing, on the edge of bankruptcy.
http://b00ks.bankhacker.com/Powersat+%28The+Grand+Tour%29/(8) Isaac Asimov, How Did We Find Out About Solar Power?
published in1981. Ever since the first person sat in the sun to get
warm, we've been tapping solar energy. The ancient Romans
disocvered how to trap the Sun's warmth in glass "greenhouses" forgrowing plants. But it wasn't until modern times that people began
to search for ways to use the Sun's light and warmth at home. As
the costs of other fuels rise, the search for solar power continues on
Earth, but its solution may lie in outer space!
http://homepage.mac.com/jhjenkins/Asimov/Books/Book230.html

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